Question

A student has to take 9 more courses before he can graduate. If none of the courses are prerequisite to others, how many groups of four courses can he select for the next semester?

Answer 1

The order does not matter. We use combination.

$$C(9,4)=\binom{9}{4}={9! \over 4!(9-4)!}={9(8)(7)(6) \over 1(2)(3)(4)}=126$$

A student can select 126 groups of four courses.