Consider the circuit in the accompanying diagram. There are four resistors: Two resistors ($10\Omega$ and $20\Omega$)connected in series on the left leg between points (a) and (c), and two resistors ($5\Omega$ and $15\Omega$) between points (b) and (d). The voltage between point (e) and the negative terminal of the battery is measured to be $15$ Volts.
What is the voltage on the battery ($V$)?
What is the voltage between point (f) and the negative terminal of the battery?