Question

A parallel plate capacitor C has a charge Q. The actual charges on the plates are​

Answer 1

This may be just a badly worded question but the answer is that it’s impossible to tell the actual charge without further information. All we can say is that the charge on the plates differs by 2Q.

All that happens when you charge a capacitor is that, starting with no charge difference, you’re removing charge from one plate and simultaneously adding the same amount to the other. This process has no impact on the base charge on the plates, and in fact this base charge could well be varying independently at the same time, and often is in a real circuit.