Checking calculations

A good way to check the subtraction is to add the answer to the number that you subtracted. You should get the number at the top of the calculation.

125.22 + 253.28 = 378.50

	1	2	5	.	\(^1\) 2	2
+	2	5	3	.	2	8
=	3	7	8	.	5	0

Checking the reasonableness of answers

Before students work on subtraction calculations, it is worth spending some time estimating answers to see if their calculations are reasonable. A strong understanding of place value is vital. Even a calculation such as 38.8462942 − 14.161836923 wouldn't be difficult for a student who understood that the estimate would be:

'about' 39 take away 'nearly' 14 is approximately 25.

In a subtraction such as 68.24 − 34.82, the numbers can be rounded to the nearest whole number.

We know that 68 and 24 hundredths is closest to 68, and 34 and 82 hundredths is almost 35.

We subtract to get: 68 − 35 = 33. With this estimation, the answer has to be near 33.

When the calculation is complete, the estimations confirm that the answer is reasonable.

	6	\(^7 8\hspace{-3mm}\color{red}/ \)	.	\(^1\) 2	4
−	3	4	.	8	2
=	3	3	.	4	2

To clearly demonstrate how to round to the nearest whole number, tenth or hundredth, the use of a number line will assist.