Chapter 2.5, Problem 2CP

The two naturally occurring isotopes of antimony, ${}^{\text{121}}\text{Sb }\left(\text{57}\text{.21 percent}\right)\text{ an}{\text{d}}^{\text{123}}\text{Sb }\left(\text{42}\text{.79 percent}\right)$, have masses of 120.904 and 122.904 amu, respectively. What is the average atomic mass of Sb?

$\begin{array}{l}\text{a})\\ 121.90\text{ amu}\\ \text{b)}\\ \text{122}\text{.05 amu}\\ \text{c)}\\ 121.76\text{ amu}\\ \text{d)}\\ \text{121}\text{.34 amu}\\ \text{e)}\\ \text{122}\text{.18 amu}\end{array}$