Explanation:
there are 91 2-digit numbers : 10 ..99
and there are 9 2-digit numbers with identical first and second digit : 11, 22, 33, 44, 55, 66, 77, 88, 99
99 has to be excluded, because there are no 2 2- digit numbers that I can subtract from each other and get 99.
so, e are dealing with 8 possibilities.
88 = 99-11
98-10
77 = 99-22
98-21
97-20
...
78-1
66 = 99-33
98-32
...
67-1
...
11 = 99-88
98-87
...
12-1
all the double- digit numbers are 11 apart. so, by picking 12 numbers, I have to have at least one that I can combine with one of the other 11 to get a double-digit number.