smc

There are $m$ oz of salt, and there is $m\times \frac{m}{100}=\frac{m^2}{100}$ oz of pure salt. We wish to add $k$ oz of pure salt so that $\frac{m^2/100+k}{m+k}=\frac{2m}{100}$. Solving this results in $k=\frac{m^2}{100-2m}\to \boxed{\frac{m^2}{100-2m}}$