I suppose it is a question in kinematics and forces, acceleration due to gravity, projectile motion.
If a particle is thrown in to the air at a projection angle Ф, with an initial speed u, it flies to a highest point and then returns to surface of earth at some distance.
Then the time for the particle to reach the highest point = u sin Ф / g
where g is acceleration due to gravity. we use v = u - g t and resolution of velocity vector along the vertical direction to solve this.
Time taken for the particle to touch ground from the highest point = u sin Ф / g
Time of flight = 2 u sin Ф / g
If the particle is thrown vertically upwards, the n duration of flight = 2u/g as Ф=90 degrees