6. There are 10 bags of coins. In nine bags, the coins are genuine (weighing 10g each), and in one bag, all the coins are counterfeit (weighing 11g each). Determine which bag contains the counterfeit coins with one weighing.
6. Let's number the bags from 1 to 10. Take one coin from the first bag, two from the second, ..., and ten from the tenth, and determine their total weight. Let this weight be $P$. If all the coins were genuine, they would weigh $10+20+\ldots+100=550$ g. The excess $P-550$ obviously coincides with the number of the bag containing the counterfeit coins.