Solid Gold!

The Bank of Puzzlania has uncovered a plot to swap the gold in their vaults with counterfeits. It was determined that all the gold bars in three of the Bank's seven vaults were replaced with counterfeits. The other four vaults were uncompromised. The plot was foiled through the poor math skills of the thieves: while the real gold bars weigh ten kilograms, the counterfeits all weighed nine kilograms. You've been tasked with determining which vaults have real gold, and which are just gold-plated bars of platinum. (The thieves were also not financial wizards.) The Bank Director has made the following generous offer to you: If you can determine the counterfeits using just one weighing on a scale, you can keep one bar as a souvenier. Here are the rules: This is a scale, not a balance, but you can weigh as many bars together as you like. Only one weighing. The bars will be handled by professional guards, so you won't have a chance to "feel" their weights. Each vault contains several hundred bars. The guards have requested that you try to keep the number of bars you need to a minimum. Good luck!