This is a classic two-set overlapping groups problem, and the key concept being tested is translating percentage relationships into the standard Venn diagram formula.
Here's how I'd break it down:
1. Set up your categories. Every student falls into exactly one of three buckets: Soccer Only, Baseball Only, or Both. There's no "Neither" here since the problem says every student opted for at least one.
2. Use the 90% clue. 90% opted for only one of the two, which means Soccer Only + Baseball Only = 90% of total. That leaves Both = 10% of total.
3. Decode "180 students have opted for baseball." This means baseball TOTAL — including those who chose both. So: Baseball Only + Both = 180.
4. Use the 50% clue. 50% of all students opted for baseball only. So Baseball Only = 50% of total.
5. Find the total. Since Baseball Only + Both = 180, and Baseball Only = 50% and Both = 10%, we get 60% of total = 180. Total = 180/0.6 = 300 students.
6. Find Soccer Only. We know Soccer Only + Baseball Only = 90% of total. Baseball Only = 50%, so Soccer Only = 40%. That gives us 40% of 300 = 120.
Answer: (A) 120Common trap: Many students read "180 have opted for baseball" and assume that's Baseball Only. It's not — it includes students who opted for both. The GMAT loves testing whether you distinguish between "total in a set" and "only in that set." Whenever a problem gives you both phrasings, pay close attention to which one includes the overlap.
Takeaway: On any overlapping sets problem, your first move should be defining whether each given number represents "only" or "total" for that category — getting this wrong is the #1 reason people miss these questions.