Why are there 90 2-digit positive integers?

Answer 1

Integers are whole numbers (including zero) and their negatives.

I = {..., -2, -1, 0, 1, 2, ...}

The positive integers are restricted to just:

P = {1, 2, 3, ...}

The two digit numbers in this set begin at 10 and end at 99. To count them, simply subtract and add one:

99 - 10 + 1 = 90.

Or, count starting at 11 and you'll end up with 89, then add one more for 10 to get 90.

:)