Question

A 9-Volt battery is used to charge a 280 mF capacitor over a 81 Ohms resistance.

How long will it take for the capacitor to reach 75% of its maximum charge?

What formula is relevant?
charging the capacitor formula;
Q=Q max (1-e ^-t/RC)

Q=qmax= 0.75
solve for t
So the Voltage is irrelevant?
Why wouldn't you use this formula?

Q=Qmax*e^(-t/RC)

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