There are 3 friends "a" "b" "c". They go to buy watch for 10 Rs each. They each give 10 Rs each. As they walk home the shop keeper realises that they were his friends and could have given them a discount. The shop keeper sends his worker with 5 Rs. The worker realises 5 Rs cant be devided into 3 people so he keeps 2 Rs to him self and gives the rest to the 3 friends.  Here is the doubt. The 3 friends get the watch for 9 Rs. 9 * 3 is 27 plus the 2 Rs = 29 RS where is the 1 Rs