The atomic mass of an element a is 63.6 atomic mass units. The only naturally occurring isotopes of element a are A-63 and A-65. The percent abundance in a naturally occurring sample of element a are closest to

X=percentage of A-63

(1-x) = percentage of A-65

We can suggest the following equation:

63x+65 (1-x) = 63.6

63x+65-65x=63.6

-2x=-1.4

x=-1.4 / (-2)

x=0.7

0.7=70/100=70%

(1-x) = 1-0.7=0.3

0.3=30/100=30%

Answer: we have 70% of A-63 and 30% of A-65.

The answer is A-63 because the atomic mass is closest to 63 than 65.