According to Ohm's law for inductance:
V = L * (di/dt) where:
V is the instantaneous voltage measured across the inductor
L is the value of inductance
di/dt is the instantaneous change in the current across the inductor
We are given that:
V = 120 volts
di/dt = 5 a/sec
Substitute with the givens in Ohm's equation to get the value of the inductance as follows:
V = L * (di/dt)
120 = L * 5
L = 120/5 = 24 Henry