Correct. Using the formula $\displaystyle{r}=\frac{{W}}{{t}}\usepackage{cmbright}\Huge\$ , you can set up an equation relating the individual and joint rates of painting: $\displaystyle\frac{{1}}{{2.5}}+\frac{{1}}{{3.5}}=\frac{{5}}{{h}}\usepackage{cmbright}\Huge\$ . Notice that 5 is the numerator in the fraction $\displaystyle\frac{{5}}{{h}}\usepackage{cmbright}\Huge\$ because the question asks how long it would take to paint 5 cars.