A projectile can have the same range $R$ for two angles of projection. If $t_1$ and $t_2$ be the times of flight in the two cases, then

$R$ is same at an angle $\theta$ and $(90°-\theta )$

$$\begin{gathered} t_1=\frac{2u \sin \theta }{g} \& t_2=\frac{2u \sin \left(90-\theta \right)}{g}=\frac{2u cos \theta }{g} \\ t_1{t}_2=\frac{2u \sin \theta }{g}\times \frac{2u cos \theta }{g} \\ t_1{t}_2=\frac{u^2 2 \sin \theta \cos \theta }{g}\times \frac{2}{g} \\ \Rightarrow t_1{t}_2=\frac{2R}{g} \\ \Rightarrow t_1{t}_2\propto R \end{gathered}$$