The answer is D (1/3).
The concept being tested here is Probability combined with counting - specifically, you need to figure out what combinations of 3 integers give a median of 5 or 6.
The 10 smallest positive integers are 1 through 10. When you pick 3 of them, the median is just the middle value when arranged in order.
1. Total ways to choose 3 from 10: C(10,3) = 10!/(3! x 7!) = 120
2. Ways to get median = 5: You need exactly one number below 5 and one above 5, with 5 in the middle.
- Numbers below 5: {1, 2, 3, 4} -- 4 choices
- Numbers above 5: {6, 7, 8, 9, 10} -- 5 choices
- Favorable for median = 5: 4 x 5 = 20
3. Ways to get median = 6: Same logic, 6 must be the middle.
- Numbers below 6: {1, 2, 3, 4, 5} -- 5 choices
- Numbers above 6: {7, 8, 9, 10} -- 4 choices
- Favorable for median = 6: 5 x 4 = 20
4. Total favorable: 20 + 20 = 40
5. P(median = 5 or 6) = 40/120 = 1/3
The trap most people fall into is trying to enumerate cases or applying inclusion-exclusion when you don't need to. Median = 5 and median = 6 are mutually exclusive events, so you can just add directly.
Also worth noting: the symmetry here is beautiful and not an accident. The integers 5 and 6 are the two middle numbers of the set 1-10, so by symmetry each has the same count (20). If you spot this during the exam you can save a calculation.
Answer: D