The efficiency of a cell = Ext. RExt.R + int. r x 100
Ext. R = X
int. r = 2Ω
Efficiency = 75% = 0.75
0.75 = XX+2
0.75( X + 2 ) = X
0.75x + 1.5 = X
0.25X = 1.5
X = 1.50.25 = 6Ω
