Saturday, January 31, 2015

Riddle/Lateral Thinking: The Fake (Counterfeit) Coin

3. Find the Fake (Counterfeit) Coin set ?


There are 5 sets of coin (10 coins in each set). All coins weight 25gms (say) each except the coins in one set which weigh 2gm more (each). You have a digital weighing machine which can be used only once.


How do you determine which set of 10 coins is of higher weight?

Note: Basic assumption holds good. 





Read it and post your answers as reply to the post. I will be adding my answer if there are no correct answers after 10 replies.  

Regards

1 comment:

Karthik Devisetty said...

If you take one ball from each bag, the weight u get would be 5*25+2=127 gms which does not suggest the bag which contains the ball which weighs 2 gms more. So, you have to consider taking different number of balls from different bags. Consider taking 1 ball from the 1st bag, 2 from the 2nd, 3 from the 3rd....... and 5 from the 5th. This means that you have taken a total of 15 balls. Ideally, all the balls are supposed to weigh 25*15=375 gms. However, as the combination contains fake balls that weigh 2 gms heavier, If the total of 15 balls weigh 377gms that implies the fake ball is from first bag. If the weight is 379gms, it implies that the fake ball is from the 2nd bag. If the total weight is 381, it implies that the fake ball is from 3rd bag. If the total weight of the balls is 383gms, it implies that the fake ball is from 4th bag. If the total weight is 385gms, it implies that the fake ball is from 5th bag.