Vitellium (Vi) has the following composition: Vi-188: 187.9122 amu; 10.861% Vi-191: 190.9047 amu; 12.428% Vi-193: 192.8938 amu; 76.711% Based on this data, what can you predict about the average atomic mass of vitellium?

multiply each isotopes mass by it's percent/fractional abundance (to get the weighted average) and add them all up, like this:

(mass of isotope #1) x (% abundance) + (mass isotope #2) x (% abundance) + ... = atomic weight (187.9122x10861) + 190.9047x12.428+192.8938x76.711=god knows what