13. From 3 boys and $n$ girls, any 3 people are selected to participate in a competition. It is known that the probability of having at least 1 girl among the 3 people is $\frac{34}{35}$, then $n=$ $\qquad$
13. 4. From the condition, $1-\frac{C_{3}^{3}}{C_{3 n+3}^{3}}=\frac{34}{35}$, which means $C_{n+3}^{3}=35$, solving this yields $n=4$.