Explanation

(b) Given, $L = 20\ mH = 20 \times 10^{- 3}H$
$C = 50\mu F = 50 \times 10^{- 6}F$
For LC circuit the frequency ,
$f = \frac{1}{2\pi\sqrt{LC}}$
or $T = 2\pi\sqrt{LC}$ $\left( \because T = \frac{1}{f} \right)$
At time$\ t = \frac{T}{4},$ energy stored is completely magnetic.
The time, $\ t = \frac{T}{4}$
$\ t = \frac{2\pi\sqrt{LC}}{4}$
or $t = \frac{2\pi\sqrt{20 \times 10^{- 3} \times 50 \times 10^{- 6}}}{4}$
or $t = \frac{3.14\sqrt{10^{- 6}}}{2}$
or $t = \frac{3.14 \times 10^{- 3}}{2}$
or $t = 1.57 \times 10^{- 3}s = 1.57\ ms$