Checking Addition and Subtraction by Estimation

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Achievement Objectives
NA3-1: Use a range of additive and simple multiplicative strategies with whole numbers, fractions, decimals, and percentages.
Specific Learning Outcomes

Solve addition and subtraction problems by using place value.

Description of Mathematics

Number Framework Stage 6

Required Resource Materials
Checking +/- by Estimating (Material Master 8-1)
Activity

Good number sense skills are required to “sense” when answers are wrong. This is often achieved by doing estimates.

Using Number Properties

Problem: “Julian adds 34 567 and 478 on his calculator and gets 34 089.” Write 34 567 + 478 = 34 089 on the board. Discuss why Julian must be wrong.
(Answer: The answer had to be bigger than 34 567 because he is adding.)
Discuss what Julian did wrong.
(Answer: Probably he pressed the subtraction button rather than the addition button.)
Problem: “Julian adds 34 567 and 478 on his calculator and this time gets 35 045. Check this answer by estimation.”
(Possible answer: 567 + 478 is a bit over 1 000 so the answer must be a bit over 35 000. So Julian’s answer looks reasonable. So Julian accepts the calculator answer.)

Examples: Worksheet (Material Master 8–1).

Understanding Number Properties:

Make up a five-digit plus five-digit subtraction of your own and explain how you would estimate the answer.

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Level Three