Eight 1 Ft coins, one of which is fake. It's a very good counterfeit, differing from the others only in that it is lighter. Using a balance scale, how can you find the fake 1 Ft coin in 2 weighings?
We put 3 1 Ft coins on one side and 3 on the other. If one side is lighter, we take the coins from that side and put them on the scale one by one. If one is lighter, it is the fake one; if they are the same, then the third one is the fake. If, however, the 3-3 coins on the first measurement had the same weight, the fake is among the remaining 2 1 Ft coins, which can be determined with a single measurement. Therefore, two measurements are always sufficient.