Let's assume the number is x. The sum of a number means addition so when you see "the sum of", it means addition. The things that are getting added are x and 5 so it becomes x+5. Then you divide x+5 by 6 and you get (x+5)/6
I added parentheses because I didn't know how to show the full fraction.
Probability of having a girl: 1/2 Probability of having another girl right after: 1/2 Probability of having a boy right after: 1/2 Then another boy: 1/2 And a third boy: 1/2
To find compound probabilities, just multiply them together.