Question

A fictitious element X is composed of 10.0 percent of the isotope 55X, 20.0 percent of the isotope 56X, and 70.0 percent of the isotope 57X. Estimate the atomic mass of element X.

Answer 1

To do this, multiply the percentage (expressed as a fraction) by the atomic number for each eg 0.1 times 55. Then add all these numbers together and that's the atomic mass for the element. I can check your answer if you like