Profit earned on selling a book for ₹450 is the same as the loss incurred on selling it for ₹390. Find the cost price of the book.

₹420

Direct concept = 

when the profit and the loss occurred is same for both the selling price of the article then the cost price of the article is the mean of both the selling prices=

Cost price = \(\frac{First SP + Second SP}{2}\) = \(\frac{450 + 390}{2}\) = 420