Recall that to compute for the emf of a circuit given current and inductance, we must recall that
[tex] emf = - M \frac{\Delta I }{\Delta t} [/tex]
where I is the current (A), M is the mutual inductance (h), and t is the time (ms). Since the current must not exceed 80.0 V, we have
[tex] 80.0 \geq 0.200(\frac{140}{t}) [/tex]
[tex] t \geq \frac{28.0}{80} [/tex]
[tex] t \geq 0.35 [/tex]
From this, we see that it must take at least 0.35 ms so it doesn't exceed 80 V.
Answer: 0.35 ms
