A particle is thrown in upward direction with initial velocity $V_0$. It crosses point P at height h at time $t_1$ and $t_2$ so $t_1 t_2$ =
$ h = V_0 t - {1 \over 2 } gt^2 $ $ \therefore {1 \over 2 } gt^2 - V_0 t + h = 0 $ $ \therefore t^2 + { 2V_0 \over g } - { 2h \over g } = 0 $ There are two real values of t .