combinations
nCr
you have n items and how many ways are there to chose r of them
nCr=[tex] \frac{n!}{r!(n-r)!} [/tex]
remember than n! means n times all numbers before like
3!=3*2*1, 5!=5*4*3*2*1
9 stations, how many ways are there to chose 6 of them
9C6=[tex] \frac{9!}{6!(9-6)!} [/tex]=
[tex] \frac{9!}{6!(3)!} [/tex]=
[tex] \frac{9*8*7}{3!} [/tex]=
[tex] \frac{9*8*7}{3*2*1} [/tex]=
[tex] \frac{3*4*7}{1} [/tex]=
84
84 ways
