Three friends go to a tea shop and order tea and snacks. The bill comes to Rs. 30. They give the waiter Rs. 30 by contributing Rs. 10 per head. The tea shop owner gives the waiter Rs. 5 and tells him to tell the customers that this was a special discount because it was the fifth anniversary of the shop. The waiter returns the five one rupee coins to the friends. They pocket one rupee each and give the remaining two coins, i.e. two rupees, to the waiter.
Now the friends paid Rs. 9 per head and that amounts to Rs. 27. The waiter got Rs. 2. That totals up to Rs. 29. Where has one rupee gone?