Answer: 9 boys
Reason:
The multiples of 13 are
13, 26, 39, 52, ...
We only need to focus on the first two since they are smaller than 35
So we focus on 13 and 26
- If there are 13 girls, then there are 35-13 = 22 boys
- If there are 26 girls, then there are 35-26 = 9 boys
The second option has more girls than boys, so that must mean there are 26 girls and 9 boys.