Correct. Thinking about the girls' rates in terms of how long it would take each of them to wash the entire fence, you can use the equation $\displaystyle\frac{{1}}{{4}}+\frac{{1}}{{6}}=\frac{{1}}{{t}}\usepackage{cmbright}\Huge\$ to represent this situation. Solving for t yields $\displaystyle{t}={2}\frac{{2}}{{5}}\usepackage{cmbright}\Huge\$ , or 2 hours and 24 minutes.