1. Two cells, X and Y, of emf and internal resistance ${ \epsilon }_{X}$,  ${r}_{X}$ and ${ϵ}_{Y}$, ${r}_{Y}$ respectively, are connected in turn to a variable resistor R.

The emf of cell Y is 50% the emf of cell X.

Which expression gives the terminal potential difference V that equalises the current in both circuits?