Capacitor calculation problem
Using the information given, we can use the following equation for the voltage at any time in a discharging capacitor:

$$V = V_0e^{-{t \over RC}}$$

Then, rearranging:

$${V \over V_0} = e^{-{t \over RC}}$$

$$\ln\left({V \over V_0}\right) = \ln\left(e^{-{t \over RC}}\right)$$

$$\ln\left({V \over V_0}\right) = {-{t \over RC}}$$

$$C = -{t \over R*\ln\left({V \over V_0}\right)} $$

And, entering your values:

$$C = - {6.1 \over 1.5*10^6 * \ln({2 \over 20})} \approx 1.77 \mu F $$