[Stuey]

[ QUOTE ]
3 groups of 4 rocks is right
if they don't weigh the same or in the case of the type of scale I am using "balance" I remove 2 rocks from each side at the same time.

If that causes the scale to balance then I know I got the bad one in one of my mitts.

If the scale stays put then I can use the 4 stones I pulled as standard weighing ones to eliminate the remaining 4 on the scale with the previous method.

Basically I am cheating and using the scale more than 3 times but it does not apear that way as I am just removing the rocks in a certian order to help my deductions.

Makes sence to me but I describe it badly.

[/ QUOTE ]

Am I wrong? here is a longer version

Separate the rocks into 3 equal piles of 4.

Step 1

Put any 2 piles on a balance type scale, if they balance the 3rd pile has rock X in it. And you can use any 2 rocks on the scale as standard weight rocks to find out where X is in the remaining 4 using step 2 below.

If they don’t balance remove 2 from each side at the same time. If that makes it balance then you know you are holding rock x and all other rocks are standard weight so you can use them to find which of the 4 in your hands is X easily with step 2.

If it does not balance when you remove 2 from each side you know rock x is on the scale and all other rocks are standard and you can use step 2 to find it.

It will look like you only use the scale 1 time but you actually use it 2 times as you remove the rocks in a certain order in the case when it does not balance on the first test. It is a cheat but you gota live!

Step 2

Now that you know which group of 4 rocks x is in just take 2 from that pile and weigh them vs 2 standard ones. If it balances take 2 of the remaining ones and weight it vs 2 standard ones and you will have found it. Just remove one of the rocks and watch the scale to find out which is X and if it is heavy or light based on the scale reaction. Then say I hold X and it is heavy or light or say rock X is on the scale and it is heavy or light whatever the case might be.

You only need to use the scale twice in all possible combinations once you narrow it down to 4 rocks and you have 2 standard ones to use. You have to pull the trick from step one here again to find out if the rock is heavy or light though but it will look like you only used the scale twice. The trick is watching what happens when you remove one rock from each side.