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Checking Addition and Subtraction by Estimation

Achievement Objectives:

Achievement Objective: NA3-1: Use a range of additive and simple multiplicative strategies with whole numbers, fractions, decimals, and percentages.
AO elaboration and other teaching resources

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)

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.