First principles (see
wiki)
Another way to explain it:
Assuming a constant 20mA current discharge of a charged capacitor, how long can a given capacitor support that current? An Ampere is a Coulomb per second, or I=Q/t. Rearranging gives Q=I*t.
Also, amount of charge Q in/out of a capacitor when it's voltage changes by ΔV is Q=C*ΔV, where C is the capacitance in Farads.
Which can be written as C*ΔV=Q=I*t. Solving for C=I*Δt/ΔV
So, your problem is to supply 20mA for 5 sec. The bigger you make ΔV, the smaller C will be.