Estimating differences, step by step
Estimating a difference means rounding both numbers first, then subtracting the friendly pair.
The question
371 − 103
371−103
370−103
370−100
270
- Do not work it out exactly. Round each number first. 371 is nearer 370 than 380, so to the nearest ten it becomes 370.
- 103 is nearer 100 than 110, so to the nearest ten it becomes 100.
- Now it is easy enough to do in your head: 370 − 100 = 270.
- The exact answer is 268, so the estimate of 270 is 2 away. Close enough to catch a wrong answer with, and you did it in your head.
How it works.
- 01
Do not work it out exactly. Round each number first. 371 is nearer 370 than 380, so to the nearest ten it becomes 370.
- 02
103 is nearer 100 than 110, so to the nearest ten it becomes 100.
- 03
Now it is easy enough to do in your head: 370 − 100 = 270.
- 04
The exact answer is 268, so the estimate of 270 is 2 away. Close enough to catch a wrong answer with, and you did it in your head.