2. Jovan and Goce were fishing with their sons. Jovan caught as many fish as his son, and Goce caught three times as many as his son. Together, they caught 25 fish. Jovan's son is named Risto. What is the name of Goce's son?
Solution. Let Jovan caught $x$ fish. His son Risto caught the same number. If Goce's son caught $y$ fish, then Goce caught $3y$ fish. Therefore, all together they caught $2x + 4y$ fish, so $2x + 4y = 25$. The last equation is not possible, because the left side is divisible by 2, while the right side is not divisible by 2.

Thus, there were two fathers and two sons at the fishing, but they were not four different people. This is only possible if one person is both a father and a son at the same time. Since Jovan and Risto are father and son who caught the same number of fish, and Goce caught three times more fish than his son, we conclude that only Jovan can be both a father and a son, i.e., Goce's son is Jovan. Indeed, in this case, we get the equation $3x + x + x = 25$, i.e., $x = 5$, which means that Jovan and Risto caught 5 fish each, and Goce caught 15 fish.