Answer
$\Delta t = 1.0~ms$
Work Step by Step
We can find the minimum time required:
$\Delta V = L~\frac{\Delta I}{\Delta t}$
$\Delta t = L~\frac{\Delta I}{\Delta V}$
$\Delta t = (0.20~H)~(\frac{2.0~A}{400~V})$
$\Delta t = 1.0\times 10^{-3}~s$
$\Delta t = 1.0~ms$
