Cells in Series and in Parallel

A wire has resistance of $12$ ohms. It is bent to form a circle. The effective resistance between the ends of any of its diameter is

A few storage cells in series are to be charged from a $200 \mathrm{V}$ D.C. supply. The emf of each cell is $2.5 \mathrm{V}$ and internal resistance $0.1 \mathrm{\Omega }$. The charging current is $8 \mathrm{A}$. In this arrangement, how many cells can be changed and what extra resistance is required to be connected in the circuit?