Use 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{8}\cdot{50}={400}\usepackage{cmbright}\Huge\$ , so k would be 400.

The second two numbers give you $\displaystyle{10}\cdot{40}={400}\usepackage{cmbright}\Huge\$ .

Because k is the same, the relationship is inversely proportional.