Answer
16
Work Step by Step
The probability of getting a 5 is $p=1/8=0.125$.
Let $k=2$ be the number of successes of getting 5s, using the
negative binomial distribution formula, we have
$\mu=\frac{k}{p}=\frac{2}{0.125}=16$
Thus, the average number of rolls it takes to get two 5s is $16$.
