By selling 100 pens, a shopkeeper gains the selling price of 20 pens. Find his gain percent.

Correct option is A

Ans. (a)
Sol.
Let the Selling Price (SP) of 1 pen be ₹1.
Then, SP of 100 pens = ₹100.
According to the question, Profit = SP of 20 pens = ₹20
Now,
Cost Price (CP) of 100 pens = SP − Profit = ₹100 − ₹20 = ₹80
Gain % = (Profit / CP) × 100
= (20 / 80) × 100
= 25%