Many people criticise the fact that children use calculators and, indeed, they have huge potential for giving incorrect answers: numbers left in the memory and mishit keys are just two ways of ensuring wrong answers. That is why it is so important that a mental calculation, no matter how brief, always goes on at the same time as the calculator is used, to ensure that the answer is ‘sensible’.
For example, a child might want to know quickly what 3 876 x 3 is. By all means use the calculator, but at the same time, he should be rounding 3 876 to 4 000 and multiplying 4 000 by 3 to make 12 000. This should take only a second or so to do in his head. He then knows that the answer will be just less than 12 000.
3 876 x 3 is actually 11 628
This is why the ability to round quickly and sensibly is so important. Hence this worksheet, found in our Year 5 Counting and Number section.