A schedules lectures of 9 professors, 3 professors every day, till all possible combinations are exhausted. No combination of professors is ever repeated on any day. How many days will each professor have to come?
a) 56
b) 28
c) 84
d) 70
Ans: b) 28
If one professor is fixed, then the two other professors for that day can be selected from the remaining 8 professors in 8C7 = 28 ways.
