Question 30
Suppose that $7$ boys and $13$ girls line up in a row. Let $S$ be the number of places in the row where a boy and a girl are standing next to each other. For example, for the row $GBBGGGBGBG$$GGBGBGGBGG$ we have $S=12.$ The average value of $S$ (if all possible orders of these $20$ people are considered) is closest to