Question

Use compatible numbers to find two estimates
396/42

Answer 1

One estimate is 10 because

• 396 is close to 400
• 42 is close to 40

So 396/42 is close to 400/40 = 10

Using a calculator or long division shows that 396/42 = 9.43 approximately which isn't too far from 10.

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Another estimate is 8 because

• 396 is close to 400
• 42 rounds up to 50, when rounding up to the nearest tens

So 396/42 is somewhat close to 400/50 = 8

In some cases, dividing by 50 is easier than 40, but I think it's better to stick to the first estimate because it's closer but also because 400 and 40 can easily be divided mentally.