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.
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: