How do you solve this math problem?

Question

On a bookshelf, there are five different mystery books and six different biographies. How many different sets of four books can Emilio choose if two of the books must be mystery books and two of the books must be biographies?

Answer 1

The choices for mystery books is 5!/3!/2! = 10
The choices for biographies is 6!/4!/2! = 15
so the answer is 10*15 = 150

Answer 2

there are 5 * 4 ways to choose 2 books from the mysterious section
and 6*5 ways to choose 2 from the other section.


total number of ways to choose : 5*4 * 6*5

Answer 3

5C2 = mystery books = 10
6C2 = biograpy books = 15
Total possibilities = 10*15 = 150