Answer: Let's assume the cost price (CP) of the book is 'x' rupees.
The market price (MP) of the book is 20% more than its cost price:
Market Price (MP) = Cost Price (CP) + 20% of Cost Price (0.20 * CP) = x + 0.20x = 1.20x
The book is sold at Rs. 216 after a 10% discount:
After a 10% discount, the selling price (SP) of the book is 90% of the market price (as he gains Rs. 216).
Selling Price (SP) = Market Price (MP) - 10% of Market Price (0.10 * MP) = 1.20x - 0.10 * 1.20x = 1.20x - 0.12x = 1.08x
Now, it is given that the book is sold at Rs. 216, so the selling price is 216:
1.08x = 216
Solving for 'x':
Divide both sides by 1.08 to find the value of 'x':
x = 216 / 1.08
x = 200
So, the cost price of the book is Rs. 200.