By selling 100 bananas, fruit seller gains the selling price of 20 bananas. His gain per cent is?

∵ Gain = (S.P. of 100 bananas) - (C.P. of 100 bananas)
⇒ (S.P.of 20) = (S.P of 100) - (C.P. of 100)
⇒ S.P.of 80 = C.P. of 100
Let C.P. of each banana = Re. 1
C.P. of 80 banana = Rs. 80
S.P. of 80 banana = Rs. 100
∴ Gain % = (20 / 80) x 100 % = 25%