How many distinct positive integers can be represented as the difference of two numbers in the set {1, 3, 5, 7, 9, 11, 13, 15, 17, 19}?
We only need to calculate these
19 -17 = 2
19 - 15 = 4
19 - 13 = 6
19 - 11 = 8
19 - 9 = 10
19 - 7 = 12
19 - 5 = 14
19 - 3 = 16
19 -1 = 18
So....9 distinct (different) positive differences are possible