Make and Justify Estimates

What is the value…

0 ? 20.95 52.4-5 -0.42.4 4.43.35

Estimating answers

A farmer has 404 cows.

128 of them have been milked.

About how many are left to be milked?

Which is a better estimate:

400 – 100 = 300 or

400 – 130 = 270

Why?

Actual answer 404-128 = 276

Estimating answers

A small lawn is 7.5m by 2.5m.

Roughly what is its area?

Which is a better estimate:

7 x 3 or 8 x 3 or

7 x 2 or 8 x 2

What are the lowest and highest values?

The answer is between 7x2=14 and 8x3=24

Actual answer 7.5x2.5 = 18.75m2

Try estimating these ...

95 Christmas trees at £4.95 each

48 people climb the Eiffel Towerat €3.80 each

12 diskettes with 1.44Mb each

28 ambulances at £12,750

£470.25

€182.40

17.28Mb

£229,500

Using long multiplication

673 x 24 ≈ 700 x 20 = 14,000

673

x 24

2692 =673x4

13460 =673x20

16152

Using multiplication facts

If 673 x 24 = 16152 what is: 673 x 2.4

= 1615.2 6.73 x 24

= 161.52 6.73 x 2.4

= 16.152

Using multiplication facts

If 26 x 19 = 494 what is: 2.6 x 1.9

= 4.94 0.26 x 19

= 4.94 0.26 x 1.9

= 0.494