enjoyable puzzles
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PuzzlesTRANSCRIPT
Title: Blind Man and CardsID: blindPuzzle: A blind man was
handed a deck of 52 cards with exactly 10 cards facing up. How
could he divide it into two piles, each of which having the same
number of cards facing up?Hint: Yet to write.Solution: He divides
the cards into two piles with 10 and 42 cards each. He then flips
all cards in the smaller pile.Title: RopeID: ropePuzzle: Rajeev is
trapped atop a building 200m high. He has with him a rope 150m
long. There is a hook at the top where he stands. Looking down, he
notices that midway between him and the ground, at a height of
100m, there is a ledge with another hook. In his pocket lies a
Swiss knife. Hmm... how might he be able to come down using the
rope, the two hooks and the Swiss knife?Hint: Yet to
write.Solution: Cut rope into 50m and 100m pieces. Tie one end of
the 50m piece to the top hook and make a noose at the other end.
Pass the 100m piece through the noose and tie its two ends.Title:
Three boxes and a RubyID: rubyPuzzle: Alice places three identical
boxes on a table. She has concealed a precious ruby in one of them.
The other two boxes are empty. Bob is allowed to pick one of the
boxes. Among the two boxes remaining on the table, at least one is
empty. Alice then removes one empty box from the table. Bob is now
allowed to open either the box he picked, or the box lying on the
table. If he opens the box with the ruby, he gets a kiss from Alice
(which he values more than the ruby, of course). What should Bob
do?Hint: Yet to write.Solution: If Bob switches his choice, he wins
with probability 2/3.Title: Choice of ThreeID: threePuzzle: In the
previous problem, Bob had to pick one of the three boxes lying on
the table. If he wished to select them with equal probability, how
could he do it by using a penny in his pocket? What if the penny
was biased?Hint: Yet to write.Solution: Toss the coin twice. Let
TH, HT and TT correspond to the three boxes. If HH, repeat.Title:
Treasure IslandID: islandPuzzle: An old parchment contains
directions to a treasure chest buried in an island: "There is an
old unmarked grave and two tall oak trees. Walk from the grave to
the left tree, counting the number of steps. Upon reaching the left
tree, turn left and walk the same number of steps. Mark the point
with a flag. Return to the grave. Now, walk towards the right tree,
counting the number of steps. Upon reaching the right tree, turn
right and walk the same number of steps. Mark this point with
another flag. The treasure lies at the midpoint of the two flags" A
party of sailors reached the island. They find a pair of tall oak
trees merrily swaying in the wind. However, the unmarked grave is
nowhere to be found. They are planning to dig up the entire island.
It'll take a month. Can they do any better?Hint: Yet to
write.Solution: Yet to write.Title: 30 CoinsID: coinsPuzzle: 30
coins of arbitrary denominations are laid out in a row. Ram and
Maya alternately pick one of the two coins at the ends of the row.
Could Maya ever collect more money than Ram?Hint: Yet to
write.Solution: The first player could pick all coins in
odd-numbered positions or all coins in even-numbered positions,
whichever set is larger in value.Title: Cake CuttingID: cakePuzzle:
Mary baked a rectangular cake. Merlin secretly carved out a small
rectangular piece, ate it and vanished! The remaining cake has to
be split evenly between Mary's two kids. How could this be done
with only one cut through the cake?Hint: Yet to write.Solution: Cut
along the line joining the centres of the two rectangles.Title:
Cube ProblemsID: cubePuzzle: Imagine a cube on a flat table,
tantalizingly balanced on one of its vertices such that the vertex
most distant from it is vertically above it. (a) What is the length
of the shortest path an ant could take to go from the topmost
vertex to the bottommost vertex? (b) What will be the projection on
the table if there is a light source right above the cube? (c) What
would be the cross-section obtained if we slice the cube along a
plane parallel to the table, passing through the midpoint of the
topmost and the bottommost points of the cube?Hint: Yet to
write.Solution: (a) Square-root of five times the side of the cube,
(b) Regular hexagon, (c) Regular hexagon.Title: Tiling a
ChessboardID: chessboardPuzzle: An 8x8 chessboard has had two of
its diagonally opposite squares removed, leaving it with sixty-two
squares. Is it possible to tile it with non-overlapping 2x1
rectangles such that all squares are covered?Hint: Yet to
write.Solution: Both squares have the same color.Title: Cubes in
CubeID: cubeslicePuzzle: Imagine a 3x3x3 solid cube. How many cuts
do we need to break it into twenty-seven 1x1x1 cubes? How about an
n x n x n cube? What if we are not allowed to stack pieces and cut
them together?
Is there a way to repeatedly hop from a small cube to an adjacent
small cube (sharing a surface) within the large 3x3x3 cube such
that there is a closed path covering all small cubes, without ever
passing through a small cube twice?Hint: Yet to write.Solution: The
central 1x1x1 cube in a 3x3x3 cube has six faces, no two of which
can be revealed in a single cut. For the general a x b x c cuboid,
we need f(a) + f(b) + f(c) cuts, where f(y) = log y / log 2 . This
can be proved by induction. Note that at any point, the "largest"
cuboid requires the most steps; others can be sliced in parallel
with this cuboid. When the "largest" cuboid has dimensions a x b x
c, there are exactly 3 choices: to slice along one of the three
sides. For a lying in the range (2^i, 2^(i+1)], any cut that
results in the new "largest cuboid" being z x b x c, where z lies
in the range (2^(i-1), 2^i], is optimal. This is because f(z) for
all these z's is identical. For example, for 10 x 34 x 98 cuboid,
any of the following could be the maximal cuboid for the remaining
steps: 5 x 34 x 100
6 x 34 x 100
7 x 34 x 100
8 x 34 x 100
10 x 17 x 100
10 x 18 x 100
10 x 19 x 100
... and so on till
10 x 32 x 100 10 x 34 x 49
10 x 34 x 50
10 x 34 x 51
10 x 34 x 52
.. and so on till
10 x 34 x 64.
If a slice can pass through exactly one existing cuboid, then the
number of slices is n3 - 1 because a slice increases the total
number of cuboids by exactly 1.Title: Forty-five MinutesID:
fortyfivePuzzle: How do we measure forty-five minutes using two
identical wires, each of which takes an hour to burn. We have
matchsticks with us. And yeah, the wires burn non-uniformly.Hint:
Yet to write.Solution: Light three out of four ends. When two ends
meet, light the fourth.Section: EasyTitle: Bigger or SmallerID:
biggersmallerPuzzle: Alice writes two distinct real numbers between
0 and 1 on two chits of paper. Bob selects one of the chits
randomly to inspect it. He then has to declare whether the number
he sees is the bigger or smaller of the two. Is there any way he
can expect to be correct more than half the times Alice plays this
game with him? Hint: Yet to write.Solution: Let the number revealed
to Bob be x. Then Bob should say "bigger" with probability x,
"smaller" otherwise.Title: Thousand JarsID: thousandjarsPuzzle:
Imagine an array of one thousand jars, labeled one thru thousand.
Jars can be colored red or black. Initially, all jars are red. The
color of a jar changes on successive days as follows: On the i-th
day, jars whose labels are divisible exactly by i, switch their
color. How many jars are red at the end of the thousandth day?Hint:
Yet to write.Solution: A number has an odd number of divisors iff
it is square.Title: Working ComputerID: workingcomputerPuzzle: A
room has n computers, less than half of which are damaged. It is
possible to query a computer about the status of any computer. A
damaged computer could give wrong answers. The goal is to discover
an undamaged computer in as few queries as possible.Hint: Yet to
write.Solution: (a) Pick a computer at random. Ask other computer
whether it is damaged or not.
(b) Yet to write a second solution.Title: Average SalaryID:
averagesalaryPuzzle: Four honest and hard-working computer
engineers are sipping coffee at Starbucks. They wish to compute
their average salary. However, nobody is willing to reveal an iota
of information about his/her own salary to anybody else. How do
they do it?Hint: Yet to write.Solution: The first engineer picks a
random k-digit integer for some large k, adds his salary to it and
writes the sum on a chit. The chit is passed around. When it
returns to the first engineer, he subtracts the k-digit
integer.Title: Number Guessing Game IID: numberguessingiPuzzle:
Shankar chooses a number between 1 and 1000. Geeta has to guess the
chosen number as quickly as possible. Shankar will let Geeta know
whether her guess is smaller than, larger than or equal to the
number. (a) What should Geeta's strategy be? (B) In a modified
version of the game, Geeta loses if her guess is "larger than the
number" two or more times. (c) What if Shankar is allowed to choose
an arbitrarily large number? Hint: Yet to write.Solution: (a)
Binary search. (b) and (c) Guess 1, 4, 9, 16, 25, and so on, to
discover k such that Shankar's number lies between k2 and (k+1)2.
Then guess k+1, k+2, k+3 and so on. On the whole, this requires
O(n) steps.Title: Number Guessing Game IIID:
numberguessingiiPuzzle: Shankar chooses a number uniformly at
random between 1 and 1000. Geeta has to guess the chosen number as
quickly as possible. Shankar will let Geeta know whether her guess
is smaller than, larger than or equal to the number. If Geeta's
guess is larger than the number, Shankar replaces the number with
another number chosen uniformly at random [1, 1000]. (a) What
should Geeta's strategy be? (b) In a modified version of the game,
Shankar gets to choose a number arbitrarily / adversarially. (c)
And finally, what if Shankar just says "equal to" or "not equal to"
when Geeta guesses (the rest of the setup remains the same)?Hint:
Yet to write.Solution: (a) Repeatedly guess g = n - n/k until
Shankar says "greater than". Then guess g+1, g+2, g+3 and so on.
The expected number of steps is k + n/2k, which is minimized for k
(n/2). (b) and (c) Yet to write. Title: Four ShipsID:
fourshipsPuzzle: Four ships are sailing on a 2D planet. Each ships
traverses a straight line at constant speed. Their journeys started
at some time in the distant past. Sometimes, a pair of ships
collides. A ship continues its journey even after a collision.
However, it is strong enough only to survive two collisions; it
dies when it collides a third time. The situation is grim. Five of
six possible collisions have already taken place and two ships are
out of commission. What fate awaits the remaining two?Hint: Yet to
write.Solution: Let z-axis denote time. Then the four trajectories
are straight lines. Three of the lines are known to be coplanar.
The fourth line intersects two of these three. Still, the sixth
collision may or may not happen!Title: f(f(x))ID: ffxPuzzle: Is it
possible to write a function int f(int x) in C that satisfies
f(f(x)) == -x? Without globals and static variables? Is it possible
to construct a function f mapping rationals to rationals such that
f(f(x)) = 1/x?Hint: Yet to write.Solution: Function f can be
defined iff we can divide non-zero integers into quadruples of the
form (a, b, -a, -b). This is because f(0) must be 0. For any other
integer "a", let f(a) = b. Then f(b) = -a, f(-a) = -b and f(-b) =
a. Now, C integers are either 32-bit or 64-bit. So it is impossible
to create such quadruples because the number of positive and the
number of negative integers are not the same!Title: Measuring
WeightsID: measuringweightsPuzzle: (a) Customers at a grocer's shop
always want an integral number pounds of wheat, between 1 pound and
40 pounds. The grocer prefers to measure wheat in exactly one
weighing with a beam balance. What is the least number of weights
he needs? (b) Customers come to a pawn shop with antiques. An
antique always weighs an integral number of pounds, somewhere
between 1 pound and 80 pounds. The owner of the pawn shop is free
to do as many weighings as necessary to ascertain the unknown
integral weight by using a beam balance. What is the least number
of weights he needs?Hint: Yet to write.Solution: (a) Four weights:
1, 3, 9 and 27. (b) Four weights: 2, 6, 18 and 54.Title: Counting
with a Magical BirdID: magicalbirdPuzzle: An evil goblin assembles
100 gnomes together. He tells them that they will be locked up into
individual cells. Each cell will have a window and a large supply
of grains. Thereafter, a magical bird will hop from window to
window, ad infinitum. Initially, the bird is white in color, and it
will switch its color from white to black and vice versa if a grain
is fed to it. The bird will be fair in the sense that it will visit
every cell infinitely often. As soon as some gnome is sure that the
bird has visited every cell, he should say "abracadabra" aloud. If
the bird has indeed visited every cell, all gnomes will be
released. Otherwise, all of them will be killed. The gnomes have
ten minutes to arrive at a strategy. How can they save themselves?
Hint: Yet to write.Solution: Yet to write.Title: Troll n GnomesID:
trollgnomesPuzzle: An evil troll once captured a bunch of gnomes
and told them, "Tomorrow, I will make you stand in a file, ordered
by height such that the tallest gnome can see everybody in front of
him. I will place either a white cap or a black cap on each head.
Then, starting from the tallest, each gnome has to declare aloud
what he thinks the color of his own cap is. In the end, those who
were correct will be spared; others will be eaten, silently." The
gnomes set thinking and came up with a strategy. How many of them
survived? What if hats come in 10 different colors?Hint: Yet to
write.Solution: The tallest gnome should declare the "parity". For
example, he could say "white" iff the number of white caps he sees
is odd.Section: ModerateTitle: Red-black SquaresID:
redblacksquaresPuzzle: Given k arbitrary points in a grid of size m
by n, is it always possible to color them either red or black such
that each row and each column is balanced? A row or column is said
to be balanced if the difference in the number of red and black
points in it is at most one.Hint: Yet to write.Solution: Thanks to
Iman Hajirasouliha for this elegant solution! Model the grid with a
bipartite graph G[X, Y], a vertex in part X for each row and a
vertex in part Y for each column. Add edges between xi and yj iff
there is a point in the corresponding place. If G has a cycle (it
must be an even cycle), we color the edges alternatively and remove
those edges, continuing this procedure we can assume that G is a
tree. When G is a tree, we can solve the problem by induction on
the number of edges as follows: remove e a leaf edge, color the
remaining edges balanced, now look at the parent of that leaf and
choose the right color for e to maintain the induction
hypothesis.Title: Four CardsID: fourcardsPuzzle: Four cards are
placed on a square table, one card at each corner. A blind gnome
and an evil goblin take turns to play the following game. The blind
gnome gets to choose a subset of the four cards and flip them
simultaneously. If all four cards are face up, he wins the game.
Otherwise, the evil goblin get to rotate the table by an amount of
his choice. Show that for any initial configuration of cards chosen
by the evil goblin, the blind gnome can win the game in a finite
number of steps with a deterministic strategy.Hint:Solution: Yet to
write.Title: And-Or GatesID: andorgatesPuzzle: Alice and Bob were
working in the hardware design lab one morning giving final touches
to their 4-bit microprocessor, when suddenly they discovered that
three of their NOT gates were malfunctioning! They opened the
inventory cupboard and received another shock: only two NOT gates
were available.
"Look. All we have is hundreds of AND and OR gates but just two
NOT's! Damn! Just six hours before the deadline. We're out of luck.
Any ideas?", asked Alice.
"Well, we obviously cannot hope to get three NOTs out of two NOTs",
replied Bob.
"I guess you're right ...", said Alice with a frown on her face and
her shoulders dropping. Then suddenly she jumped and said, "No,
wait! It can be done! Here it it!" And pulling out a pen from her
pocket, she sketched a circuit of a piece of paper.
"Wow! You're a genius, Alice!", cried Bob.
Are you as smart as Alice? In general, how many signals can we
invert using n NOT gates and any number of AND and OR gates? No
other gates may be used.Hint: Yet to write.Solution: Yet to
write.Title: Dijkstra's ProblemID: dijkstraPuzzle: There are n+1
processors named 0, 1, ..., n. Processor i has a counter C(i) that
takes values in the range [0, n]. Its initial value is arbitrarily
chosen from [0, n]. Processor 0 is said to be privileged if C(0) =
C(n). Processor i, where i > 0, is said to be privileged if C(i)
is not equal to C(i-1).
At successive clock ticks, exactly one out of possibly several
privileged processors is arbitrarily chosen and its counter is
updated as follows:
If processor 0 is chosen, we set C(0) = (C(0) + 1) mod (n+1).
Otherwise, we set C(i) = C(i+1).
Prove that after a bounded number of clock ticks, exactly one
processor will be privileged. And that this will continue to hold
forever.Hint: Yet to write.Solution: Yet to write.Title: Tiling
ProblemID: tilingPuzzle: A big rectangle is composed of smaller
rectangles, each having an integral width or integral height or
both. Does the big rectangle enjoy the same property?Hint: Yet to
write.Solution: Yet to write.Title: Marked SquaresID:
markedsquaresPuzzle: Consider an n by n grid of squares. A square
is said to be a neighbour of another one if it lies directly
above/below or to its right/left. Thus, each square has at most
four neighbours. Initially, some squares are marked. At successive
clock ticks, an unmarked square marks itself if at least two of its
neighbours are marked. What is the minimum number of squares we
need to mark initially so that all squares eventually get
marked?Hint: Yet to write.Solution: Yet to write.Title: Horses on
AuctionID: horsesauctionPuzzle: You are the chief guest at an
auction, where an unknown number of horses will be revealed and
auctioned, one after the other, in no particular order. You are a
connoiseur of horses, and can judge whether one horse is 'better'
than another. Being the chief guest, you have a one-time privilege
of selecting a horse, after it is revealed, but before it gets
auctioned off. You get to keep this horse for yourself. Your
objective is to maximize the probability of selecting the best
horse. What do you do?Hint: Yet to write.Solution: Strategy: Pick
the first horse that is superior to all horses that have been
revealed so far. Analysis: If the very first horse is the k-th best
horse, then the probability of picking the best horse with this
strategy is 1 / (k - 1). The overall probability of picking the
best horse is approximately [c + log (n - 1)] / n where c is
Euler's constant. One could extend this idea to wait even more,
i.e., pick the s-th horse that beats all horses that went by.
Identifying the optimal value of s is an interesting exercise. Is s
= 1 optimal?Title: Red CardID: redcardPuzzle: Alice repeatedly
draws a card randomly, without replacement, from a pack of
fifty-two cards. Bob has a one-time privilege to raise his hand
just before a card is about to be drawn. If the card drawn is Red
just after Bob raises his hand, Bob wins; otherwise he loses. Is
there any way for Bob to be correct more than half the times he
plays this game with Alice? Hint: Yet to write.Solution: No, Bob
cannot be right more than half the times. Yet to write proof.Title:
Geometry without a RulerID: norulerPuzzle: Using only a compass
(and without a straight edge or a ruler), is it possible to
identify (a) the midpoint of two points? (b) the center of a
circle? (c) all four corners of a square, given two of them?Hint:
Yet to write.Solution: Using only a compass, all those points can
be constructed which can be constructed using a compass and a
ruler. Surprising, isn't it! The constructions are far more
complicated though. Google search for further
information.Title: Chessboard with SignsID: chessboardsignsPuzzle:
An 8x8 chessboard has either a plus or a minus written in each of
its sixty-four squares. You are allowed to repeatedly choose a 3x3
or a 4x4 block of squares and invert all signs within it. How would
you go about getting rid of all the minuses? Hint: Yet to
write.Solution: There are 36 3x3 blocks and 25 4x4 blocks. An
arbitrarily long sequence of inversions is equivalent to inverting
some subset of these 36 + 25 = 61 blocks in any order. Thus at most
2^61 distinct chessboard positions can be derived if we start with
all minuses.Title: Five Card TrickID: fivecardtrickPuzzle: A
mathemagician asks a volunteer to give him five cards drawn from a
pack of fifty-two. He hands one card back to the volunteer and
arranges the remaining four in some sequence he chooses. He then
hands the sequence to a second volunteer and leaves the room. His
assistant enters. The assistant asks the second volunteer to read
out aloud the sequence handed to him. The assistant ponders a
little and correctly announces the identity of the card held by the
first volunteer. How could this be done? In general, how large a
deck of cards can be handled if n cards are drawn initially? Hint:
Yet to write.Solution: Yet to write.Section: Challenging