Two batteries of emfs $6.20 \mathrm{V}$ and $12.45 \mathrm{V}$and internal resistances $0.01 \mathrm{\Omega }$ and $0.03 \mathrm{\Omega }$ respectively are being charged by a battery charger which supplies a current of $20 \mathrm{A}$. What are the terminal potential differences of the batteries during charging?