The atomic mass<span> of an element is a weighted average of its </span>isotopes<span> in which the sum of the abundance of each isotope is equal to 1 or 100%.
</span><span>Let fraction of N-15 be x then fraction of N-14 = 1-x </span> <span>Formula for Calculating Isotope: </span>
on calculation:
x = <span>0.006403 so, % = 0.006403 x 100 = 0.6403 %<span> </span></span>