Question

A bag contains 15 marbles numbered from 1 to 15. Two marbles are selected from the bag at the same time. Let event E = the two numbers do NOT differ by 10 How many outcomes are in the complement of E?

Answer 1

possible event will be \

1,2 or 1,3 or , 1,4  or 1,5  or  1, 6 or 1, 7 or 1,8 or 1,9 or 1, 10
 = 9

and so on....   9 + 8+  7+ 6+ 5 +4 +3 +2 +1 
 = 45