How do you find the average when maximum and minimum values are known?

You add the maximum and minimum values, then divide it by 2.

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You can't.

Let's say you have three numbers. The maximum is 10, the minimum is 1.

If the numbers are 1, 7, 10, the average is (1+7+10)/3 =6.

If the numbers are 1, 4, 10, the average is (1+4+10)/3 =5.

So two different sets of numbers with the same maximum and minimum have different averages!