A shopkeeper earns a profit of 21%after selling a book at 21% discount on the printed price. The ratio of the cost price and selling price of the book is:

79 : 100 100 : 79 121 : 100 100 : 121

100 : 121

Cost price : Selling price = (100) : (100 + Gain%)                                     = (100 ) : (100 + 21%)                                     = (100) : (121)