A shopkeeper gained Rs 8 by selling a pen by allowing 10% discount. Hw would have gained Rs 20, if he had not allowed discount. What was the cost price of the pen?

Cost of the pen = Rs.100

Step-by-step explanation:

Let the marked price = x

When the shopkeeper is selling for x, his gain = Rs. 20

Cost price = Marked price  - Gain = x - 20 (I)

After discount:

Discount = 10%

Marked price after discount = (100 - 10)% of x = 90% of x

                                               = 0.9 * x = 0.9x

When the shopkeeper is selling for 0.9x, his gain = Rs.8

Cost price = Marked price - gain = 0.9x - 8   (II)

From (I) and (II)

x - 20 = 0.9x - 8

x   = 0.9x - 8 + 20

x = 0.9x + 12

x - 0.9x = 12

0.1x = 12

x = 12/0.1

x = 120

Marked price of pen = Rs.120

Cost price = 120 - 20

Cost price  = Rs. 100