From Kirchhoff's voltage law, the formula would be:
EMF E = terminal voltage + voltage across 'internal resistance[R]' of generator
Using Ohm's Law, v = ir, let set up a pair of simultaneous equations:
E = 113 + 13.6R would be eq 1
and
E = 91 + 25.3R would be eq 2
Rearrange these:
(E - 113) / 13.6 = R
(E - 91) / 25.3 = R
(E - 113) / 13.6 = (E - 91) / 25.3
25.3(E - 113) = 13.6(E - 91)
25.3E - 2858.9 = 13.6 E - 1237.6
25.3E - 13.6E = 2858.9 - 1237.6
11.7E = 1621.3
E = 1621.3 / 11.7
E = 138.57 v
