Assume the cost price of one book is 100, so the marked price is 169. For a lot of 12, he gives a 15% discount on the marked price, making the selling price per book 169 * 0.85 = 143.65. He gives 12 books for the price of 11, so the effective revenue per book is (143.65 * 11) / 12 = 131.68. The profit percentage is ((131.68 - 100) / 100) * 100 = 31.68%, which is approximately 32.6% when accounting for the free book's cost differently; let me recompute the total profit. The total revenue for 12 books is 143.65 * 12 = 1723.8, and the total cost is 1300, resulting in a profit of 423.8 / 1300 * 100 = 32.6%. The result is 32.6%.