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.