Series and parallel circuits — Calculate

How do you calculate the combined e.m.f. of several sources connected in series?

The combined e.m.f. is the sum of the individual e.m.f.s, provided all the sources are connected in the same direction (e.g. two 1.5 V cells in series give 3.0 V); if a source is connected in reverse, its e.m.f. is subtracted from the total instead.