Weigh three and three at the same time. If they balance, the light coin is the one left out. If not, then set aside the heavier three 9and the extra) and put one of the lighter three on each side of the scale. Again, if they balance, the one you left out this time is the lighter; if not, the lighter one on the scale is the light coin.Today's logic problem is a guest problem, supplied by discordian:
Imagine a bridge over a river. A group of four travellers comes to one side of the bridge. The bridge is the only way to cross the river, but it is very narrow and weak. Also, it is pitch black out, and they only have one flashlight between them, and thus they can only cross two at a time. Due to various infirmities, the men will cross at different speeds, meaning any pair can only travel at the speed of the slowest man. The times it takes each man to cross are as follows:
Guy1: 1 minute
Guy2: 2 minutes
Guy3: 5 minutes
Guy4: 10 minutes.
Get them all across to the other side in 17 minutes.
EDIT: Just to be sure you understand, the fact that it is pitch black out means that anyone can only cross with the use of the flashlight...so it'll have to go back and forth.