Element M has two isotopes,^104M and ^106M.

The relative atomic mass of M is 104.4. What is the relative abundance of ^106M?

A:10%

B:20%

C:30%

D:40%

To calculate atomic mass, you have to take to weighted average of the isotopes' masses. What that means is M = RA*106 + (1 - RA) * 104, where RA is relative abundance expressed in decimal form. If you simplify the right side of that equation, you get M = 2*RA + 104. Doing a little more algebra yields RA = (M - 104) / 2 = (104.4 - 104) / 2 = 0.4 / 2 = 0.2, which is 20%. So the answer is B.