Explanation:

- The shopkeeper buys two books for a total of Rs. 300.

- Let the cost price of the first book be Rs. x. Therefore, the cost price of the second book will be Rs. (300 - x).

- The first book is sold at a profit of 20%. So, its selling price is 1.2 times its cost price, which is 1.2x.

- The second book is sold at a loss of 10%. So, its selling price is 0.9 times its cost price, which is 0.9(300 - x).

- The transaction results in no profit, no loss. So, total selling price should be equal to the total cost price.

- Therefore, 1.2x + 0.9(300 - x) = 300.

- Solving: 1.2x + 270 - 0.9x = 300.

- 0.3x = 30.

- x = 100.

- Selling price of the first book is 1.2 * 100 = Rs. 120.

- Answer: Rs. 120