A shopkeeper allows people to buy a bottle of cold drink in exchange of five empty bottled. A guy had a party at home and he managed to spend 77 cold drink bottles in the party. Now he has 77 empty bottles with him.

How many bottles do you think he can buy in exchange of those 77 empty bottles ?

He can purchase 19 new bottles in return.

He will get 15 bottles after returning 75 empty bottles and will be left 2 empty bottles.
Now, the 15 bottles when empty will get him 3 more bottles.
The three empty bottles will get him another bottle. Remember he had 2 empty bottles as well.