[ Average values ]

Three pirates divided the diamonds they had obtained during the day in the evening: twelve each for Bill and Sam, and the rest for John, who could not count. At night, Bill stole one diamond from Sam, Sam stole one from John, and John stole one from Bill. As a result, the average weight of Bill's diamonds decreased by one carat, Sam's decreased by two carats, but John's increased by four carats. How many diamonds did John get?
The number of diamonds each pirate had did not change over the night. Since Bill had 12 diamonds, and their average weight decreased by 1 carat, the total weight of his diamonds decreased by 12 carats. Similarly, the total weight of Sam's diamonds decreased by 24 carats. Since the total weight of Bill's and Sam's diamonds decreased by 36 carats, the weight of John's diamonds increased by the same 36 carats. Since the average weight of his diamonds increased by 4 carats, he had $36: 4=9$ diamonds.

## Answer

9 diamonds.