Question

A penny falls from a windowsill which is 25.0 m above the sidewalk. How much time does a passerby on the sidewalk below have to move out of the way before the penny hits the ground?

Answer 1

You need to know the formula:

25 = 1/2 * g * t^2

t = √(50/g) = 2.258 s, where I use g = 9.8 m/s^2