Given,Let the cost price of single book be Rs. 100.

The cost price of (9 + 1) = 10 pair i.e. 20 books = Rs. (100 × 20) = Rs. 2000.He gets profit of 26%.

So, the selling price of 9 pair i.e. 18 books = Rs. 2000 × (126/100) = Rs. 2520

Then,the selling price of single book = Rs. 2520/18 = Rs. 140

He gives 20% discount on the marked price of a book.

That means, when the selling price is Rs. 80 then the marked price is Rs. 100.

∴When the selling price of single book is Rs. 140, the marked price = Rs. 140 × (100/80) = Rs. 175

∴The percentage increase in marked price from the cost price = (175 –100)% = 75%

Let Principal = Rs. P and Rate = R% p.a. Then,

Amount= Rs.P1+R1004

 C.I = P1+R1004-1

 P1+R1004-1=1491

Clearly, it does not give the answer