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

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

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%

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

Answers are 1,2,4 its correct on edgenuity.