Great Problem Solving question testing Overlapping Sets — the key is recognizing what "at least" means and setting up the constraint correctly.
Key concept: Overlapping Sets with a minimization condition.
Common trap: Students often jump to thinking the minimum number of interns is exactly 35 + 30 = 65. That's wrong — it ignores the overlap. Once you account for the overlap correctly, the algebra gives a tighter bound.
Step-by-step:
1. Define variables. Let N = total number of interns. We know: Women (W) = 35, Economics majors (E) = 30, Women who are economics majors (W ∩ E) = 10% of N = 0.10N.
2. Apply the inclusion-exclusion principle. W ∪ E = W + E − (W ∩ E) = 35 + 30 − 0.10N = 65 − 0.10N.
3. The union can't exceed the total. Since W ∪ E ≤ N, we get: 65 − 0.10N ≤ N → 65 ≤ 1.10N → N ≥ 65/1.1 = 59.09... So N ≥ 60 (rounding up since N must be a whole number). Minimum N = 60.
4. Find the minimum fraction of men. Men = N − 35 (everyone who isn't a woman). The fraction of men = (N − 35)/N = 1 − 35/N. This fraction is minimized when N is smallest (since 35/N is largest when N is smallest). So plug in N = 60: Men fraction = (60 − 35)/60 = 25/60 = 5/12.
5. Verify: If N = 60, then W ∩ E = 6, W = 35, E = 30. W ∪ E = 35 + 30 − 6 = 59 ≤ 60. ✓ Also W ∩ E = 6 ≤ min(35, 30) = 30. ✓
Answer: B (5/12).
The takeaway: whenever a question asks for the "minimum fraction" of a group, set up your constraint inequality first, find the boundary value of N, and then compute the fraction at that boundary — don't try to guess from the answer choices.