Answer :
[tex]111-91=20-error\\\\\begin{array}{ccc}91&-&100\%\\\\20&-&p\%\end{array}\ \ \ |cross\ multiply\\\\\\91p=100\cdot20\\91p=2000\ \ \ \ |divide\ both\ sides\ by\ 91\\\\\boxed{p\approx21.98\ (\%)}\leftarrow answer[/tex]