Explanation:
Let relative ratio of one isotope (62.94 u) be X
Then, relative ratio of other isotope (64.93) will be (1 - X)
Now,
(62.94)x + (64.93)(1 - x) = 63.55
1.99x = 1.38
X = 0.69
Relative Abundance :
(62.94 u) isotope = 69 %
(64.93 u) isotope = 31 %