How many two-digit numbers have digits that add up to 10?

Answer 1

To make the list of two-digit numbers with digits that add up to 10, first write the pairs of single-digit integers that add up to 10. They are (1,9), (2,8), (3,7), (4,6), (5,5). Then recognize that each of these pairs of numbers, except 5, can be combined in two different ways to make two-digit numbers that satisfy the problem.

19 and 91

28 and 82

37 and 73

46 and 64

55

So there are nine two-digit numbers with digits that add up to 10.