Let the % of isotope with at. wt. 10.01 = x
\(\therefore \) % of isotope with at. wt. 11.01 = (100 - x)
Now, since,
At. wt. = \(\frac{x \times 10.01 + (100 - x)\times 11.01}{100}\)
\(10.81 = \frac{x \times 10.01 + (100 - x)\times 11.01}{100}\)
x = 20
Hence
% of isotope with at. wt. 10.01 = 20
% of isotope with at. wt. 11.01 = 100 - 20 = 80