Antimony (atomic number 51) is composed of two principal isotopes, antimony-121, which has a mass of 120.904 atomic mass units, and antimony-123, which has a mass of 122.904 atomic mass units. Assume that 57.36% of all antimony atoms are the antimony-121 isotope and 42.64% are the antimony-123 isotope.
What is the atomic mass of natural antimony?