Three CD's are bought at an average cost of $\$ 15$ each. If a fourth $\mathrm{CD}$ is purchased, the average cost becomes $\$ 16$. What is the cost of the fourth $\mathrm{CD}$ ?
(A) $\$ 16$
(B) $\$ 17$
(C) $\$ 18$
(D) $\$ 19$
(E) $\$ 20$
Three CD's are bought at an average cost of $\$ 15$ each. If a fourth $C D$ is purchased, the average cost becomes $\$ 16$. What is the cost of the fourth $\mathrm{CD}$ ?
(A) $\$ 16$
(B) $\$ 17$
(C) $\$ 18$
(D) $\$ 19$
(E) $\$ 20$

## Solution

If four C.D.'s have an average cost of $\$ 16$ this implies that $\$ 64$ was spent in purchasing the four of them. Using the same reasoning, $\$ 45$ was spent buying the first three. Thus, the fourth C.D. must have cost $\$ 64-\$ 45=\$ 19$.

ANSWER: (D)