Q:

Find the starting height from which a particle projected horizontally at $40$m/s travels $100$m

This question is in my homework: A particle is projected horizontally with speed 40m/s from a point A. It hits the ground 100m horizontally from A. Find the height of A.
Would it be possible for someone to guide me on how to start? $g=9.8$

A:

First find the time it takes for the projectile to travel $100$ m horizontally, given a horizontal speed of $40$ m/s.  Call this time $t$.  Then use the formula $d = \frac{gt^2}{2}$ to obtain the vertical distance that the projectile falls in that time.

