I am going to choose 6 students out of the 15 who currently have A's to be mentors for the students who have C's or lower. How many ways can a group of 6 be selected?
ok look :o
\[15!=15\times14\times13\times12\times11\times......\times1 \]
and\[9!=\times8\times7\times6\times5\times...1 and 6!=6\times5\times4\times3\times2\times1\]
so if you sub this in, we can easily simplify the equation
\[(15\times14\times13\times12\times11\times10\times9!)\div(6!(9!))\]
you can cancel out the 9! and be left w/\[(15\times14\times13\times12\times11\times10)\div(6!)\]
then just simplify by using intuition