Make and Justify Estimates

1
Make and Justify Estimates
2
What is the value
-0.4
2.4
4.4
3.35
0
?
2
0.95
5
2.4
-5
3
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

4
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 7x214 and 8x324
• Actual answer 7.5x2.5 18.75m2

5
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
6
Using long multiplication

• 673 x 24 700 x 20 14,000
• 673
• x 24
• 2692 673x4
• 13460 673x20
• 16152

7
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

8
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