An LC circuit has L = 0.50 H, C = 20 µF and a resistance of 2.0 Ω…

Step 1. The time. The amplitude falls to half when $e^{-Rt/2L} = \dfrac{1}{2}$. Taking the natural logarithm, $\dfrac{Rt}{2L} = \ln 2$. We want $t$, so $t = \dfrac{2L\ln 2}{R} = \dfrac{2 \times 0.50\ \text{H} \times 0.693}{2.0\ \Omega} = 0.347$ s.

Step 2. The energy. The energy is proportional to the square of the charge amplitude. So it falls to $\left(\dfrac{1}{2}\right)^2 = \dfrac{1}{4}$, which is $25\ \%$.

Step 3. The oscillations. $f = \dfrac{1}{2\pi\sqrt{LC}} = \dfrac{1}{2\pi\sqrt{0.50\ \text{H} \times 20 \times 10^{-6}\ \text{F}}} = 50.3$ Hz. In $0.347$ s there are $50.3\ \text{Hz} \times 0.347\ \text{s} = 17.4$ oscillations.