Determine the atomic mass of each element, given the isotopic composition.
a. Lithium, which is 92.4% lithium-7 (mass 7.016 u) and 7.60% lithium-6 (mass 6.015 u)
b. Oxygen, which is 99.76% oxygen-16 (mass 15.995 u), 0.035 oxygen-17 (mass 16.999 u), and 0.205% oxygen-18 (mass 17.999 u)