An element X has two isotopes, $^{65}X_{30}$ and $^{66}X_{30}$ with relative abundance of 60 % and 40% respectively. The relative atomic mass of X is

66.00

65.50

65.00

65.40

Correct answer is D

Isotope $^{65}X_{30}$ (with a mass of 65 and an abundance of 60%):
Relative mass contribution = (65 * 60%) = 39.00
Isotope $^{66}X_{30}$ (with a mass of 66 and an abundance of 40%):
Relative mass contribution = (66 * 40%) = 26.40
Now, add the relative mass contributions of both isotopes:
Relative atomic mass $A_r$ = 39.00 + 26.40 = 65.40
So, the relative atomic mass of element X is 65.40. The correct option is:
65.40

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