Bunuel
A reading list for a humanities course consists of 10 books, of which 4 are biographies and the rest are novels. Each student is required to read a selection of 4 books from the list, including 2 or more biographies. How many selections of 4 books satisfy the requirements?
A. 90
B. 115
C. 130
D. 144
E. 195
We can break this problem into scenarios:
Scenario 1: 2 biographies and 2 novels
The number of ways to select 2 biographies is 4C2 = 4!/[2! x (4 - 2)! = 4!/(2! x 2!) = (4 x 3)/2! = 6.
The number of ways to select 2 novels is 6C2 = 6!/[2! x (6 - 2)!]= 6!/(2! x 4!) =(6 x 5)/2! = 15.
So there are a total of 6 x 15 = 90 ways for this scenario.
Scenario 2: 3 biographies and 1 novel
The number of ways to select 3 biographies is 4C3 = 4.
The number of ways to select 1 novels is 6C1 = 6.
So there are a total of 4 x 6 = 24 ways for this scenario.
Scenario 3: 4 biographies
The number of ways to select 4 biographies is 4C4 = 1.
So there is only 1 way for this scenario.
So there are a total of 90 + 24 + 1 = 115 ways to choose 4 books that consist 2 or more biographies.
Answer: B