Correct. Using the formula $\displaystyle{r}=\frac{{W}}{{t}}\usepackage{cmbright}\Huge\$ , you added the rates of the individual machines to equal the total rate of production. $\displaystyle\frac{{1}}{{30}}+\frac{{1}}{{50}}=\frac{{1}}{{x}}\usepackage{cmbright}\Huge\$ , so x = 18.75 (or 18 minutes and 45 seconds).