Consider a resistor (R=1000 kΩ) and a capacitor (C=1μF) connected in series. This configuration is connected in series to a battery with an emf of 10 V.
After the capacitor is charged to its maximum value, the capacitor and resistor are disconnected from the battery and from each other. Then they are connected in parallel, so that the capacitor discharges through the resistor. How long will it take for the voltage across the capacitor to drop to 2.50 V?