Use the fact that rate equals work done over time, or $\displaystyle{r}=\frac{{W}}{{t}}\usepackage{cmbright}\Huge\$ . The machine's rate is $\displaystyle\frac{{75}}{{15}}\usepackage{cmbright}\Huge\$ donuts per minute. The fraction $\displaystyle\frac{{t}}{{500}}\usepackage{cmbright}\Huge\$ does not express the machine's rate for the new job.