A shopkeeper has 11 books of same cost price. He sells the first book at 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 11 books. If he sells sixth book all percent profit or loss on selling all 11 books = ?

Explanation:-

(x - 5), (x - 4) ........., x(cost price of 6th article), (x + 1)(x + 2) ....... (x + 5)
Average = ^{(Price of 1st book + Price of 11th book)}/₂ $$\eqalign{ & = \frac{{x + 5 + x - 5}}{2} \cr & = x \cr}$$ Total cost price of 11 books = 11x
Total selling price of articles = 11x
Both are equal so neither profit nor loss