Question

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

Answer 1

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

Answer 2

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.