Chapter 31 - Electromagnetic Oscillations and Alternating Current - Problems - Page 935: 2a

At $~~t = 5.0\times 10^{-6}~s~~$ plate A will again have maximum positive charge.

We can find the period: $T = \frac{1}{f} = \frac{1}{2.0\times 10^5~Hz} = 5.0\times 10^{-6}~s$ At $~~t = 5.0\times 10^{-6}~s~~$ plate A will again have maximum positive charge.