Expected value with empirical probabilities

Find expected value based on empirical probabilities.

Problem

Divya spends all of her free time playing with her building blocks. She owns building block sets with 3 pieces, 5 pieces, and 10 pieces.
Divya knows she owns 5 sets with 5 pieces and 2 sets with 10 pieces. She also knows that she averages 5 pieces per set. This information is summarized in the table below.
Fill in the missing blank with how many sets with 3 pieces Divya owns.
Pieces in a setSets
3
55
102
Get 3 answers correct in a row