These numbers satisfy a directly proportional relationship. They need to satisfy an inverse variation equation: $\displaystyle{t}=\frac{{k}}{{r}}\usepackage{cmbright}\Huge\$ or $\displaystyle{t}\cdot{r}={k}\usepackage{cmbright}\Huge\$ .

The first two numbers give you $\displaystyle{10}\cdot{10}={100}\usepackage{cmbright}\Huge\$ , while the second two numbers give you $\displaystyle{20}\cdot{20}={400}\usepackage{cmbright}\Huge\$ . Because k is not the same, the relationship is not inversely proportional. The correct answer is 50 mph for 8 hours and 40 mph for 10 hours.