A battery of e.m.f. $15 \mathrm{V}$ and internal resistance $3 \mathrm{\Omega }$ is connected to two resistors $3 \mathrm{\Omega }$ and $6 \mathrm{\Omega }$ connected in parallel. Find the p.d. between the terminals of the battery.

The following circuit diagram shows three resistors $2 \mathrm{\Omega }$, $4 \mathrm{\Omega }$ and $R \mathrm{\Omega }$ connected to a battery of e.m.f. $2 \mathrm{V}$ and internal resistance $3 \mathrm{\Omega }$. A main current of $0.25 \mathrm{A}$ flows through the circuit. Find the p.d. across the $4 \mathrm{\Omega }$ resistor.