The numbers DO differ by 10 if they are
- 1 and 11
- 2 and 12
- 3 and 13
- 4 and 14
- 5 and 15
Each outcome technically has two ways of occurring, but since we're taking two marbles at a time, that would be the same as saying, for instance, that drawing 1 and 11 is the same as drawing 11 and 1. We only count such an event once.
So the answer is (a) 5.
This is a word problem signifying an algebraic expression.
Translating this to mathematics:

add 4 to both sides.
divide 16 to both sides.
this is:
42/6 is 7 because seven 6s go in 42
9 squares because if there are 18 triangles in it then you divide 18 by 2 and get 9