2  [ Problems with constraints ]

From a class of 28 students, 4 people are appointed to duty in the cafeteria.

a) In how many ways can this be done?

b) How many ways are there to form a team of duty students that includes Kolya Vasin, a student from this class?
б) If Kolya is already chosen, then we need to choose 3 people from the remaining 27.

## Answer

a) $C_{28}^{4}=20475$ ways; b) $C_{27}^{3}=2925$ ways.