formula of permutation P(n,r) n!/(n-r)
for example P(9,4) 9!/(9-4) 9*8*7*6*5*4*3*2*1/4*3*2*1
crash out the 4*3*2*1 of the numerator and the 4*3*2*1 of the dinamenator and (×) multiply the remaining 9*8*7*6*5 and the answer is15,120 

1 2 1
19,958,400.  I'm sure you it's just a typo error, because you explained clearly how to solve the factorial of n (n!).
^"i'm sure it's just a typo error..."