The pilot of an airplane traveling 190 km/h wants to drop supplies to flood victims isolated on a patch of land 100 m below.
how many seconds before the plane is directly overhead should the supplies be dropped?

All we need to calculate is the time it takes for the supplies to fall from the plane to the ground. Using the free-falling body equation,

y = -1/2*gt^2 + y00 = -1/2*9.80*t^2 + 100t = 4.52 s.

The supplies should be dropped 4.52 s before the plane is directly overhead.