We need to sell 5 books.
Least price at which book can be bought is 400.
Greatest price at which it can be sold is 1200.
Least Cost Price = Rs. (400 * 5) = Rs. 2000.
Greatest Selling Price = Rs. (1200 * 5) = Rs. 6000.
Required profit = Rs. (6000 - 2000) = Rs. 4000.