find the internal resistance of the cell

Question

two cells each of emf 1.5 v are joined in series and send 1 A current in an external resistor.if the same cells are joined in parallel then they send 0.6 A current in the same resistor .what is the internal reistance of the cell?

Answer 1

2E = I (R + 2r)

2E = 1 (R + 2r)

R + 2r = 2*1.5

R + 2r = 3 

R = 3-2r ……(1) 

E = 0.6 (R+r/2) 

1.5 = 0.6(R+r/2)

R+r/2 = 1.5/0.6 = 2.5 ......(2)

Equating equation (1) and (2)

3-2r+r/2 = 2.5

-3r = -0.5*2 = -1

r = 1/3 = 0.333 ohm