A shopkeeper has 11 books of same cost, price. he sells

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A shopkeeper has 11 books of same cost, price. he sells the first book at a certain price, then he sells second book at a price which is Rs. 1 less than the selling price of first book and then he sells third book at a price which is Rs. 1 less than the selling price of second book. Following this pattern, he sold all books. if he sells sixth book at its cost price. Find the overall percent profit or loss on selling all books?

a) 20%
b) 10%
c) 9 1/11%
d) 0%