A professor gives his students 7 essay questions to prepare

Reginald Hanna

Reginald Hanna

Answered question

2022-06-02

A professor gives his students 7 essay questions to prepare for an exam. Only 5 of the questions will actually appear on the exam. How many different exams are possible?

Answer & Explanation

blogrom1j7hf8

blogrom1j7hf8

Beginner2022-06-03Added 4 answers

Step 1
Of the 5 questions, the first question can be chosen in 7 ways. Having chosen one question out of 7, the second question can be chosen in 6 ways, third question in 5 ways, the fourth question in 4 ways and the fifth question in 3 ways.
So the total number of ways of choosing 5 questions from 7 is 7 × 6 × 5 × 4 × 3 = 7 ! ( 7 - 5 ) ! .
Step 2
This can be generalized as follows:
Permutations: The number of ways of choosing r objects from a collection of N objects is P ( N , r ) N ! ( N - r ) !
Answer: P ( 7 , 5 ) = 7 ! ( 7 - 5 ) ! = 2520

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