$\begin{align}  & x\propto \frac{1}{y} \\ & x=\frac{k}{y} \\ & \text{when }x=2\tfrac{1}{2},\text{ }y=2 \\ & \frac{5}{2}=\frac{k}{2} \\ & k=5 \\ & \text{when }y=4 \\ & x=\frac{k}{y}=\frac{5}{4}=1\tfrac{1}{4} \\\end{align}$