Estimating sums and products, step by step
Rounding is a skill; estimating is what rounding is for.
The question
161 + 708
161+708
160+708
160+710
870
- Do not work it out exactly. Round each number first. 161 is nearer 160 than 170, so to the nearest ten it becomes 160.
- 708 is nearer 710 than 700, so to the nearest ten it becomes 710.
- Now it is easy enough to do in your head: 160 + 710 = 870.
- The exact answer is 869, so the estimate of 870 is 1 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. 161 is nearer 160 than 170, so to the nearest ten it becomes 160.
- 02
708 is nearer 710 than 700, so to the nearest ten it becomes 710.
- 03
Now it is easy enough to do in your head: 160 + 710 = 870.
- 04
The exact answer is 869, so the estimate of 870 is 1 away. Close enough to catch a wrong answer with, and you did it in your head.