Checking Addition and Subtraction by Estimation

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.