Weighty problem

[Space cat]

You have twelve coins. They appear identical but you have reason to believe that one is faulty; it is the wrong weight. You have a pair of scales (beam and pans only, no weights) which you are allowed to use just three times.

Your mission, should you choose to accept it, is to identify the faulty coin (if any) and say whether it is overweight or underweight. Can you do it?

 

[toasty]

Yes!!

I won't spoil it, but I really like this puzzle...
I got asked it at an interview for Oxford once (I couldn't answer it, but had a go)

-Dan

 

[blondini]

Weigh 6 v 6 then take the lighter 6 and weigh them 3 v 3. Then take two coins from the lighter 3 and weighh 1 v 1. If neither of them is the light coin then it's the odd one you didn't weigh.

 

[blondini]

Call me a spoilsport but it was easy and I didn't know it in advance.

 

[holmslaw]

You put 4 coins on each side - if level.
add 1 more coin on each side - if level add the the 2 remaining coins - thus showing the dodgy coin.

If at stage 1 above the scales are not level - remove two coins from each side. If still not level, remove 1 coin from each side this will show the dodgy coin. If when you remove the first two coins and it's level you replave 1 coin on each side which show the dodgy coin.

 

[noseall]

not this again.

 

[solo]

Are the scales on a conveyor belt?

 

[blondini]

Call me a spoilsport but it was easy and I didn't know it in advance.

Easy to get wrong didn't read the question properly...

 

[Steve]

Are the scales on a conveyor belt?

Sorry lol, of all the conveyor belt jokes lately this one made me crack up, great timing.

 

[Space cat]

not this again.

Oops! I should've searched more carefully.

Since bethrob99 et al have already solved that one, here's round two:

You have thirteen coins ---- You also have one extra coin known to be good - and without which you can't do it. Your new mission - should you choose to accept it - etc, etc, etc.

It can be done but thirteen is the absolute limit. For bonus marks, tell us why.

 

[solo]

You still haven't said if there's a conveyor belt. You'll probably get a better response if you include a conveyor belt. People who visit this forum like conveyor belts.

 

[tim west]

Conveyors suck!

 

[noseall]

down with conveyors!