By selling 100 pens, a shopkeeper gains the selling price of 40 pens. Find his gain percent.

(SP of 100 pens) - (CP of 100 pens) = SP of 40 pens
⇒ SP of 60 pens = CP of 100 pens.
Let CP of each pen = ₹ 1
Then, CP of 60 pens = ₹ 60
SP of 60 pens = ₹ 100
Gain = 100 - 60 = ₹ 40
∴ Gain % = (40/60) x 100 % = 66.67%