A shopkeeper cheats to the extent of 10% while buying and selling fruits, by using tampered weights. His total gain, in percentage, is :

While buying he takes 10% more goods than he pays for, and while selling he charges 10% more than the goods he gives. Each cheat multiplies his money by 1.1, so overall he gets 1.1 × 1.1 = 1.21 times his cost – a gain of 21%.