An element has three naturally occurring isotopes. ten percent (.10) occurs as 55^x, fifteen percent (.15) occurs as isotope 56^x, and seventy-five percent (.75) occurs as 57^x. calculate the weighted atomic mass of element x to the nearest tenth

The average atomic weight is, from the name itself, the average weight of all its naturally occurring isotopes. All you have to do is multiple the abundance of each isotope with its individual mass, then add them altogether.

Mass = (0.10*55)+(0.15*56)+(.75*57)
Mass = 56.65 amu