The reason you use factorials is if you had 11 cans to put on the shelf and 11 spots to put them in, in the first spot you could put 11 different cans in that first sport. Now you have one can on the shelf and for the second spot you could put 10 different cans in that spot. Now you have two cans on the shelf, so for the third spot you could put 9 different cans. And so on. So it ends up being 11! = 11*10*9*8*7*6*5*4*3*2*1
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