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You add the two numbers, and divide the result by 2.
There are no whole numbers between 20 and 21. Only fractions or decimals exist between these two numbers. Whole numbers between 20 and 25 are 21, 22, 23, and 24.
Write both numbers as decimal numbers, then look for terminating decimals between the two.
Decimal numbers are infinitely dense throughout their domain. That means that between any two decimals there are infinitely many decimals. One consequence is that there is no smallest or first such number.
Line up the decimal points and then subtract one from the other.