A single battery fo emf E and internal resistance r is equivalent to a parallel combination of two batteries of emfs E1=2V and E2=1.5V and internal resistances r1=1Ω and r2=2Ω respectively, with polarities as shown in the figure.
(IIT-JEE 1997)
E=56V
Simple,
All you have to do is resolve the cell combination.
E=E1r2−E2r1r1+r2=2×2−1.5×12+1
=2.53=56V