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{4}\cdot{2.5}={10}\usepackage{cmbright}\Huge\$ , while the second two numbers give you $\displaystyle{3}\cdot{3.5}={10.5}\usepackage{cmbright}\Huge\$ . Because k is not the same, the relationship is not inversely proportional.