smc

If Carlos took $70\%$ of the pie, there must be $(100-70)\%=30\%$ left. After Maria takes $\frac{1}{3}$ of the remaining $30\%,1-\frac{1}{3}=\frac{2}{3}$ of the remaining $30\%$ is left. Therefore, the answer is $30\%\cdot \frac{2}{3}=\boxed{20\%}.$