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.