solve this ?

a person 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 ?

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Lets say the mark price is X

then on selling 12 books he gets (0.8X)*11 = 8.8X

Lets say the cost price of each book is Y then the total cost of 11 books is 12Y

Now he made 10% profit thus 1.1*(12Y) = 8.8X

12Y = 8X

X = 3/2Y = 1.5Y

Thus X is 50% more than the Y, hence mark price is 50% more than the cost price.