In identical calls are joined in series with its two cells `A` and `B` in loop with reversed polarties. `EMF` of each shell is `E` and internal resistance `r`. Potential difference across cell `A` or `B` is (here `n gt 4`)

Answer

`2E (1 - (2)/(n))

Explanation

In a series combination of identical cells each of emf `E` if one cell is connected with reverse polarity, if will reduce the effective emf with of all cell by `2E` but total internal resistance of cell will remain unchanged
As two cells are connected with reverse polarity in the combination of cell , their effective emf
`= n E - 4E = (n - 4) E`
Total internal resistance of the cell `= nr`
Current in the circuit `I = ((n - 4)E)/(nr)`
potential difference across `A ` or `B` is
`V =E + I r = E = ((n - 4)Er)/(nr) = 2E (1 - (2)/(n))`