Hello,
For a simple circuit like this where we have a voltage source charging a capacitor through a fixed resistor we can use the following formula:
Vc=Vs*(1-e^(-t/RC))
where
Vc is the capacitor voltage at time t,
Vs is the fixed source voltage,
t is time,
RC is the RC time constant equal to R*C.
The problem stated however is not clear. The cap is charged up to 500v but then it is charged up to 450v, so which is it or is it really discharging?
You'll have to clarify a bit here.