Checking Subtraction (III)


In this article let us see how we can estimate the answer for the given subtraction.

Checking by Estimation: This we have discussed earlier for checking addition. The same logic is applicable for subtraction.

In the real life situations this is very useful, as we are supposed to manage our sums faster without paper and pencil. For example, after purchasing something the payments we make (if paid in cash) are usually little higher than that we are supposed to make. Obviously, we have to check, if we have got back the dues. In such an event the ability to estimate the difference quickly is very useful.

We can estimate subtraction in two ways. Here is the first one.

Estimating the difference by rounding An approach that a child may use in estimating a subtraction, illustrated for 53 - 28, is as follows: Seeing that the difference must not exceed 30(60 - 30) or 30(50 - 20) provides a higher limit for the answer. The lower limit for the answer is 20, since the minuend is more than 5 tens and the subtrahend is less than 3 tens. Note that in both the cases the answer is not exact (which is 25 in this case). In estimating the answer above, note that at first we rounded both the minuend and the subtrahend to the higher bound. Thus, the minuend was 60 and the subtrahend was 30. Whereas, second time we rounded both, the minuend and the subtrahend to the lower bound. This time the minuend was 50 and the subtrahend was 20. It is a good idea to show our children what happens if one (either the minuend or the subtrahend) is rounded to the lower bound and the remaining to the higher bound. This is shown in figure 1, with four different examples.

Studying the above examples one can easily note that rounding both, the minuend and the subtrahend, to either the higher bound or the lower bound simultaneously, gives a better estimate of subtraction.

On the other hand if the subtrahend is rounded to the higher bound or rounded up and the minuend is rounded to the lower bound or rounded down; we get lowest limit of the difference (highly underestimated answer). Whereas, if the minuend is rounded to the higher bound or rounded up and the subtrahend is rounded to the lower bound or rounded down we get the highest limit of the difference (highly overestimated answer).

The copyright of the article Checking Subtraction (III) in Math for Kids is owned by Vidya Narayan Wadadekar . Permission to republish Checking Subtraction (III) in print or online must be granted by the author in writing.

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