When cells are connected in series, their voltages add up. However, if two cells are connected in the wrong direction, they will subtract from the total e.m.f If all cells were connected correctly, the total e.m.f. would be: Total E.M.F = 8 x 2.0V = 16V ( since no of cell = 8. and E.M.F of each cells = 2.0V) When two cells are connected in the reverse direction, their contributions effectively subtract from the total e.m.f Contribution from 2 wrongly connected cell = 2 x 2.0v = 4V Therefore, Total E.M.F = 16 - 4 = 12V.