Question: 2015 Fall Midterm - 17
Let $S = \{1,2,3,4,5,6,7\}$. You choose a uniformly random 3-element subset $X$ of $S$. Thus,
each 3-element subset of $S$ has a probability of $\left. 1 \middle/ {7 \choose 3} \right.$ of being $X$.
Define the event
- A = "4 is an element of $X$".
(a)
7/3
(b)
15/7
(c)
7/15
(d)
3/7