I was a history major......


http://i148.photobucket.com/albums/s...z/coinstar.jpg

My idea:

#1-Divide the 100 coins into piles of 50. The heavier pile has the counterfeit.

#2-Divide the heavier pile into piles of 25. The heavier pile has the counterfeit.

#3-Divide the heavier pile into two piles of 12 (24). You hold one coin.
IF the piles are equal, you have the counterfeit!
IF they are unequal, you have a "normal" coin, and the heavier pile
has the counterfeit.

#4-Take the heavier pile (unless you were lucky above) of 12, and divide it into 6. The heavier pile has the counterfeit.

#5-Take the heavier pile of 6 and divide into two groups of 3. The heavier pile has the counterfeit.

#6-Take the heavier pile of 3, and put one coin on each pan. You hold one coin.
IF the coins are equal-you have the counterfeit in your hand.
IF the coins are unequal-that heavy coin is the counterfeit.

You would separate the coins into four groups of 25. Put group #1 on one side of the scale and weigh group #2 against it.

1) First, we need to isolate a control group that has all good coins in it.

Case A) If they weigh the same, neither has the bad coin. weigh group #3 against #1. If #1 and #3 are the same, then #4 is the culprit.
If they are different, #3 is the culprit.

Case B) If they weigh different, weigh group #3 against group #1. If they are different also, then #1 is the culprit.

Take 5 coins from the good coins to be your control group and put the rest of the good coins aside to purchase ice cream and dark chocolate.

2) Whittle down some more.

Break the culprit group into groups of 5.

Put your control group on one side of the scale.
Measure each of the culprit groups against the control group, stopping as soon as you find a group that is different.

Save one coin from the control group and the culprit group that was different.
Put the rest of the coins towards that new dildo you were saving for.

3) Go for the kill.

Take one coin from the control group and measure it, one by one, against the coins
from the new culprit group.

As soon as you find a coin that does not weigh the same, you have found the counterfeit.

Take the control coin and the other good coins and save up for some nice cologne for the boi who just found the counterfeit coin for you...and, please, remember to share the chocolate, ice cream, and dildo with the boi as well.

;)

I knew I was bad with navigation here... I didn't see the other answers first! .... quick, where's my tom-tom?!! LOL

I'd just use my hands and see if there was any discernible difference before spending money on a scale. Then, I would compare it against another coin of the same denomination for a control.

I think people should remember the accuracy of their statements are so easy to check these days with all the modern technology available!

This is a variation of an interview question that I ask. I believe the optimal algorithm is (3) piles of 33 with one coin leftover. Do you have the actual answer?

Is Clyde correct?

I have NO idea but if it's safe to travel you can bet I'd be on that damn train!