A (current vs time) graph of the current passing through a solenoid is shown in the fig. For which time is the back electromotive force (u) a maximum? If the back emf at t = 3s is e, find the emf at t = 15s

Question

(a) e
(b) 2e
(c) $\frac {e}{2}$
(d) $\frac {-e}{2}$