Puzzle of the week - Week #3
There are ten coin machines. Each machine produces coins that weigh 10 g, expect one faulty machine which produces coins that weigh 9 g. You are given with an electronic weighing scale. You can take any number of coins from any machine and use the weighing scale. What would be the minimum number of usage of the weighing scale to find out the faulty machine?
Solution: Take 1 coin from the 1st machine, 2 coins from the 2nd, 3 coins from the 3rd and so on. Total number of coins are 1+2+3+…10, which is (10*11)/2 i.e. 55. The theoretical weight should be 550 g. Weigh it and find the practical weight and let it be x g. Now (550-x)/10 gives you the position of the machine that is faulty.
Comment by praveen — January 28, 2006 @ 7:47 am