The areas of three squares are 16,49 and 169. What is the average (mean) of their side lengths?
(A) 8
(B) 12
(C) 24
(D) 39
(E) 32
Since the areas of the three squares are 16,49 and 169 , then their side lengths are $\sqrt{16}=4$, $\sqrt{49}=7$ and $\sqrt{169}=13$, respectively.

Thus, the average of their side lengths is $\frac{4+7+13}{3}=8$.

ANSWER: (A)