Math CentralQuandaries & Queries


Question from Erika, a parent:

When working with compatible numbers, when do you choose to round up or down? For example: In the problem: 373 divided by 4, one could round 373 to 400 and keep the 4 the same, and the answer would be 100; OR, one could round 373 down to 360 and keep the 4 the same, and the answer would be 90. When given the problem as an assignment, which way dictates how one would round, up or down?

Hi Erika,

The choice isn't whether to round up or down, it's to chose the compatible number that's closer to the original number. In your example of 373, to round up to 400 you move 400 - 373 = 27 units and to round down you move 373 - 360 = 13 units. Hence rounding down will give you a closer approximation.

Even these subtractions you can approximate using compatible numbers. When deciding whether to move from 373 to 360 or 400 you can approximate 373 by 370 and immediately see that moving up to 400 is about 30 units and moving down to 360 is about 10 units, so move down to 360.

I hope this helps,

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