The atomic mass of element a is 63.6 atomic mass units. the only naturally occurring isotopes of element a are a-63 and a-65. the percent abundances in a naturally occurring sample of element a are closest to
a) 31% a-63 and 69% -65
b) 69%a-63 and 31% a-65
c) 50% a-63 and 50% a-65
d) 100% a-63 and 0% a-65

This question uses weighted averages.

Since the average mass of element A is 63.6 amu and the only two isotopes are 63 and 65, 

63.6 = x% * 63 + (1-x%) * 65

If you don't want to calculate this (like me), you can look at the answer choices.

If it was 100% A-63, the weight would be 63. Cross off D.
If it was 50/50, the weight would be 64. Cross off C.
So since the weighted average is 63.6, you would expect there to be more A-63 than A-65, so B is the only answer that makes sense.

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