Let us assume that the boy has fallen freely, without any friction (without holding the rope tightly) during 15/3 = 5 meters. He was at rest (speed = 0) at the ceiling of the gymnasium. let his mass be m = 50 kg.
The potential energy corresponding to the drop of 5 meters is converted in to the kinetic energy, due to conservation of (mechanical) energy.
Drop in P.E. = m * g * 5 = gain in kinetic energy = KE - 0
So K.E. = 50 * 9.81 * 5 = 2452.5 Joules (it is 2,500 J if g = 10 m/sec²)