If a boy of a mass 50 kg runs up a flight of stairs in 3s If the flight is 3m high calculate his gain in potential energy?

Mechanical potential energy, U, equals mgh where m is mass, g is the gravitational constant, and h is the change in height from the reference point.

Applying U=mgh this situation, you get:

U=(50 kg)(9.8 m/s2)(3 m) = 1470 Joules (J)