Answer
$7.494\%\ and\ 92.506\%$
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 $x$.
$6.0152x+701601(1-x)=6.941$
Simplify and we get $x= 0.07494$
2. Determine the percent relative abundance from each isotope.
% abundance of lithium-6 = $x=0.07494\times100\%=7.494\%$
% abundance of lithium-7= $(1-x)\%=92.506\% $
