Answer
$19.903\% \ and\ 80.097\%$
Work Step by Step
1. The relative abundance of both isotopes add to $100\%$. Therefore, denote one isotope with an abundance of $x$ and the other abundance as $1-x$. Then set-up an equation representing the weighted average and then solve for $x$
$10.01294x+11.00931(1-x)=10.811$
Simplify and we get $x= 0.19903$
2. Determine the percent relative abundance from each isotope.
% abundance of boron-10 = $x=0.19909\times100\%=19.903\%$
% abundance of boron-11= $(1-x)\%=80.097\% $
