Checking the reasonableness of answers
Sometimes it can be easy to make a simple error in calculations involving addition. It is important to use knowledge of place value to check whether your answer is reasonable.
With a simple calculation such as 5.7 + 2.1, we round off each number to the nearest whole number and then add to give an estimate. 5.7 is rounded to 6 and 2.1 is rounded to 2. So our estimate is 6 + 2 = 8.
The use of a number line helps to make this more evident:
When you do the calculation, you find you are correct.
| 5 | . | 7 | |
| + | 2 | . | 1 |
| 7 | . | 8 |
If the numbers being added involve hundredths, the number line can clearly demonstrate how to round to the nearest whole number:
34.87 + 21.17 =
34.87 is rounded to 35 and 21.17 is rounded to 21.
35 + 21 is equal to 56.
When we do the calculation, we can see that the estimate is close to the answer:
| 3 | 4 | . | 8 | 7 | |
| + | 2 | \(1_1\) | . | \(1_1\) | 7 |
| 5 | 6 | . | 0 | 4 |
  |
This publication is funded by the Australian Government Department of Education, Employment and Workplace Relations |
Terms of use  |
