Physical Science
posted by Stephanie .
A 9Volt 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 (1e ^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)