Assume that you have a 20 volt source available and that this 20 volt source is very good, that is it has a negligible internal resistance.  Such a source is usually referred to as a "stiff" source.  Design a circuit that gives 10 volts open circuit with no more than 100 ohms internal resistance.  That is, your circuit should be equivalent to the following.  Your circuit should also consume the smallest amount of power possible.

        Repeat the design when the 20 volt source is replaced by a stiff 25 volt source.