Find at what % above cost price is the marked price ?

Question

A bookseller sells book at a discount of 20% on the mark price and he gives 1 book free on very 11 books brought and still makes a profit of 10%.
Find at what % above cost price is the marked price ?

Answer 1

Let's set the market price at 100 (can be any number, but 100 calculates easier).
He sells for 80. Of every 12 books he sells 11, making him 880.
His profit is 10%, so the cost price for the 12 books is 800.
The cost price per book is then 66.67.
The market price profit margin is then 100/66.67 = 1.5 -> 50% above cost price.

Answer 2

mark price is 51.25% above cost price.

if we calculate by taking mark price as RS 100 then Cost is 66.1157, means profit is 33.8843