How to calculate equivalent emf of a cell ?

Be careful here; the emf of the cell is the electrical energy given to each coulomb of charge passed through the cell, and is usually a starting point ("this is a cell with an emf of 2.05 volts") and not an answer to be found.
The p.d. of the cell is the energy of each coulomb of charge that leaves the cell; this is not the same as the emf as a real cell has an internal resistance and the charge loses energy as it passes through the internal resistance.  
I didn't say this would be easy...
The linking equation is
V = E - Ir
where V = p.d. of cell (what the voltage seems to be when you measure it with an external voltmeter)
E = emf of cell (the voltage across the cell when no current is passing through it)
I = current through the cell
r = internal resistance of the cell
so
you'd need a couple of circuits drawn for you, and some information about V, I & r , to get E.