Question: how many combinations with numbers 1-10?
Answer: You can do that in 20 ways as follows. (5,10) (6,9) (7,8) (1,4,10) (1,5,9) (1,6,8) (2,3,10) (2,4,9) (2,5,8) (2,6,7) (3,4,8) (3,5,7) (4,5,6) (1,2,3,9) (1,2,4,8) (1,2,5,7) (1,3,4,7) (1,3,5,6) (2,3,4,6) (1,2,3,4,5). The sum of first 9 digits is 45.