[input]7![/input]
first customer can be placed at 7 different positions in the line.
second customer can be placed at (7-1)=6 different positions in the line. (because first customer already occupies a position).
and so on.
that's: 7 (possibilities first customer) * 6(possibilities second customer) * 5*4*3*2 * 1 (possibility last customer) = 1*2*3*4*5*6*7 = 7!
+3146 
