39778

I used the method " Guess and check" and "Process of Elimination" to get the answer. But I did it in a systematic way. I first wrote down all the sums made up of two numbers that makes 15. Then I used the second number of the equation and a new number that makes 20. Then I do the same thing until I get some answers. I got two answers which were

1) 6, 9, 11, 12, 4, 14, 7

2) 8, 7, 13, 10, 6, 12, 9