smc

Let $a$ be the original number of ounces of acid and $w$ be the original number of ounces of water. We can write two equations since we know the percentage of acid after some water and acid. $$\frac{a}{a+w+1}=\frac{1}{5}$$ $$\frac{a+1}{a+w+2}=\frac{1}{3}$$ Cross-multiply to get rid of the fractions. $$5a=a+w+1$$ $$3a+3=a+w+1$$ Solve the system to get $a=1$ and $w=3$. The percentage of acid in the original mixture is $\frac{1}{1+3}=\boxed{25\%}$.