Question 220726
I agree with your son, even if the answer doesn't make sense. I reckon the question is badly set up. 

Let's call the number of students in (only) honors pre-alg H, the number of students in (only) math enrichment M and the number in both B. We've got:

1) H + B = 100
2) B + M = 30
3) H + M = 105

1 + 2)  2B + H + M = 130
1 + 2 - 3) 2B = 25

B = 12.5

Edit: I disagree with Jim's answer - his working would be correct if there were 105 students in Cluster C altogether.