When current changes from 14 A to 6 A in 0.5 sec through a coil, the emf induced is 4×10^-4 . The coefficient of self induction is:

25x10^-6H

Self induce emf is given by e = -Ldi/dt

                               4x10^-4 = -L [(6-14)/0.5]

                                          = -L (-8/0.5)

                                       L = (4x10^-4x 0.5)/8

                                          =  25x10^-6H