Math CentralQuandaries & Queries


Question from Katie, a parent:

I am having trouble remembering how to find compatible numbers used to find the estimate in the following problem. Can you please help me solve it and explain it to me? My son is in 5th grade and I need help. Thank you. Here is the problem: 2,752 / 28 estimate: 90


Compatible numbers are numbers that are close to the original numbers and ones that make the arithmetic easy. When I see 2,752/28 I immediately think that 28 is close to 30 and it's also close to 25 and each of these might make the arithmetic easy. If I choose to use 30 then I would like the numerator to be divisible by 3. The numerator 2,752 is close to 2,700 and 27/3 = 9 so the answer expected is

2,752/28 is approximately 2,700/30 = 90

There are other "correct" answers to this question. For example 2,752 is close to 2,800 so you might say

2,752/28 is approximately 2,800/28 = 100


2,752/28 is approximately 2,750/25 = 110, but this is probably asking too much of someone in 5th grade.

I hope this helps,

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