You may have thought that the sums of the two numbers should be constant. However, these numbers 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{60}={600}\usepackage{cmbright}\Huge\$ , while the second two numbers give you $\displaystyle{40}\cdot{30}={1},{200}\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.