27. The names of three friends are Kostya, Vasya, and Kolya. Their surnames are Semyonov, Burov, and Nikolaev. Whose surname is whose - is unknown. But it is known that Semyonov's grandfather is a biological brother of their neighbor Petrov, Kostya is a year older than Kolya, and Kolya is a year older than Nikolaev. The sum of their ages is more than 49 but less than 53. Kolya's mother is the daughter of the well-known Professor Korobov. Determine the surnames of the friends and the age of each.
27. From the condition of the problem, we immediately conclude that Nikolaev's name is Vasya. Next, we note that Kolya's mother is Korobova, and therefore the mother of the third boy, Kostya, must have a maiden name different from Korobova. However, Semyonov's mother's maiden name is not Korobova, but Petrova, since her father - Semyonov's grandfather - is Petrov. Therefore: Semyonov is Kostya and Burov is Nikolay. The age of each of the friends can be found as follows: let $x$ be Vasya's age, then Kolya's age is $(x+1)$ years and Kostya's age is $(x+2)$ years. The sum of the ages of the three friends is $3x+3$. We get the inequality: $49<3x+3<53$. From this,

$15 \frac{1}{3}<x<16 \frac{2}{3}$, so $x=16$. Therefore: Vasya is 16 years old, Kolya is 17, and Kostya is 18.