Really need help..
A fictitious element X is composed of 10.0 percent of the isotope 55^X , 20.0 percent of the isotope 56^X , and 70.0 percent of the isotope 57^X . Estimate the atomic mass of element X.
To find the solution for this, calculate the weighted average:
(10×55+20×56+70×57) / (10+20+70) = 56.6 amu
.
+4116 
