Mass Spectra of Elements Practice Test

An element R has two isotopes that produce peaks at $m/z=79$ (relative abundance 50) and $m/z=81$ (relative abundance 50) on a mass spectrum. What is the average atomic mass of element R?

A mass spectrum of element Y shows three isotopic peaks at $m/z=24$ (relative abundance 79), $m/z=25$ (relative abundance 10), and $m/z=26$ (relative abundance 11). Assuming $+1$ charge for each ion, which value is closest to the average atomic mass of Y?