multiplying whole numbers and decimals lesson 2-2
TRANSCRIPT
Things you need to know…
• Do not line up the decimal points!
• Pretend all decimals are whole numbers until the very end– Only look at the decimals when trying to
figure out where to put the decimal point in your answer
Strategy #1405 • 59 = x
405
x 59or
405
x 50
405
x 9+
45
5 x 9 = 45Put the 5 in the answer and carry the 4 (it’s really 40) to the next place value (tens)
4
(9 x 0) + 4 = 4
Write down the 4.
36
(9 x 4) = 36
Since there are no other numbers to multiply, write down the 36.
Zero is a place holder. Put a zero in your answer to hold
its place.
025
5 x 5 = 25Put the 5 in the answer and carry the 2 (it’s really 20) to the next place value (tens)
(5 x 0) + 2 = 2
Write down the 2.
02(5 x 4) = 20
Since there are no other numbers to multiply, write down the 20.
2
Strategy #2The Lattice
405
x 59
There are 3 digits in this number
There are 2 digits in this number
We will start by making a
3 x 2 window…
Strategy #2The Lattice
405
x 59
4 0 5
5
9
x
Multiply one number from the top with one from the side. Put your answer in the box where they meet.
4 x 5 = 20
20
Strategy #2The Lattice
405
x 59
4 0 5
5
9
x
20
00 2
5
36 0
04
5
Now we will add numbers diagonally from right to left…
What if I have a decimal point in my problem?
• Multiply your numbers using one of the steps just shown.
• When done, count all the numbers after a decimal point in your problem.
• Make sure your answer has exactly that number of digits in the decimal spaces.
If my problem has decimals…
4.05
x 5.923,89
5
4.05
x 5.9
405
x 59
23.895
.405
x .059
.405
x .059
.023895
I had to add a zero because there are six total decimals spots in my problem, but I only had 5 digits in my answer. Notice that the zero came between the decimal point and my old answer.
Strategy #2The Lattice
395
x 84
There are 3 digits in this number
There are 2 digits in this number
We will start by making a
3 x 2 window…
Strategy #2The Lattice
3 9 5
8
4
x
Multiply one number from the top with one from the side. Put your answer in the box where they meet.
8 x 3 = 24
24
395
x 84
Strategy #2The Lattice
3 9 5
8
4
x
24
72 4
0
12 3
62
0
Now we will add numbers diagonally from right to left…
395
x 84
What if I have a decimal point in my problem?
• Multiply your numbers using one of the steps just shown.
• When done, count all the numbers after a decimal point in your problem.
• Make sure your answer has exactly that number of digits in the decimal spaces.
If my problem has decimals…
39.5
x 8.4
33180
39.5
x 8.4331.8
0
.395
x .084
.395
x .084
.033180
I had to add a zero because there are six total decimals spots in my problem, but I only had 5 digits in my answer. Notice that the zero came between the decimal point and my old answer.
395
x 84
78.6 x 983
7
9
8
8 6
3
63
72
54
56
64
48
21
24
18
83
1
6
1
2
2
7
17
77263.8
There is one digit after a decimal point.
772638
.392 x .008
3
0
0
9 2
8
00
00
00
00
00
00
24
72
16
631
1
3
0
0
.003136
There are 6 digits after a decimal point.
003136