Use the fact that rate equals work done over time, or $\displaystyle{r}=\frac{{W}}{{t}}\usepackage{cmbright}\Huge\$ . Henry's rate for the previous job was $\displaystyle\frac{{400}}{{2}}\usepackage{cmbright}\Huge\$ square feet per hour. The expression $\displaystyle{1},{500}{t}\usepackage{cmbright}\Huge\$ does not express his rate for the new job.