Independent solution

How to solve this Expected Value question

Setup

Setup

Let q be the probability that one of the possible item pairs is selected.

q=Pr(some pair is selected)q=\Pr(\text{some pair is selected})

Model

Model

Whenever a pair is selected, exactly two marginal item indicators equal one. Therefore the sum of the three marginal probabilities is 2q.

Pr(A)+Pr(B)+Pr(C)=2q\Pr(A)+\Pr(B)+\Pr(C)=2q

Compute

Compute

The marginal probabilities sum to one, so 2q=1 and q=1/2.

2q=14+13+512=1,q=122q=\frac14+\frac13+\frac5{12}=1,\qquad q=\frac12

Answer

Answer

The complementary probability that no pair is selected is also 1/2, which is choice C.

1q=12(C)\boxed{1-q=\frac12\quad\text{(C)}}