Element X consists of two isotopes with masses of 62.9 and 64.9.
The relative atomic mass of the element is 63.6. The percentage
abundance of the lighter isotope is closest to -
Select one:
-65%

-55%

-35%

-45%

65%.

Explanation:

If the abundance of the lighter isotope is  x then the abundance of the heavier isotope is 1-x. So:

62.9x + 64.9(1 - x)  = 63.6

62.9x - 64.9x = 63.6 - 64.9

-2x = -1.3

x = 0.65   or 65%.

16

explanation:

the marks go by 4s