Since each intern gets at least one task, the only ways to split
5 tasks among
3 interns are:
3-1-1 or
2-2-1. We need to count the arrangements for each split.
Counting 3-1-1 (one intern gets 3 tasks):- Choose which intern gets
3 tasks:
3 ways
- Choose which
3 of the
5 tasks they get:
C(5,3) =
10 ways
- The remaining
2 tasks go one each to the other
2 interns:
2! =
2 ways
- Total =
3 ×
10 ×
2 =
60Counting 2-2-1 (one intern gets 1 task, two get 2 each):- Choose which intern gets just
1 task:
3 ways
- Choose which
1 of the
5 tasks they get:
5 ways
- Split the remaining
4 tasks into
2 groups of
2 for the other
2 interns:
C(4,2) =
6 ways (
note that no need to divide by
2 since the interns are distinct people)
- Total =
3 ×
5 ×
6 =
90Total valid arrangements =
60 +
90 =
150Probability that one intern gets
3 tasks =
60 /
150 =
2/5Answer: DKey Principle: When
distributing distinct items to distinct people with constraints, list all valid partitions first, then count each partition carefully by choosing WHO gets the special role, WHICH items they get, and HOW the rest are distributed.