Video: Adding Two Numbers up to 99 by Estimation

Which number is nearest to 30 + 19? [A] 50 [B] 30 [C] 80 [D] 40.

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Video Transcript

Which number is nearest to 30 plus 19? Is it 50, 30, 80, or 40?

Do you notice anything about these numbers: 50, 30, 80, and 40? They all end in zero, which means they’re all multiples of 10. We can see that one of the numbers in our question, 30, is also a multiple of 10. So that must mean that we have to make our second number, 19, also a multiple of 10. So we need to work out which multiple of 10 is nearest to 19.

19 has one 10 and nine ones. If we add one to the ones digit, we would end up with two tens and no ones, 20. If we round 19 to the nearest 10, we get 20. Now we can write our calculation, 30 plus 20. Three plus two is five. Zero plus zero is zero. 30 plus 20 equals 50.

The number which is nearest to 30 plus 19 is 50. We rounded 19 to the nearest 10, which was 20, and calculated that 30 plus 20 is 50.

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