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Since order does matter, we must use the
permutation formula:


Start with the given formula


Plug in

and


Subtract

to get 6
Expand 15!

Expand 6!



Cancel


Simplify


Now multiply 15*14*13*12*11*10*9*8*7 to get 1,816,214,400
So 15 choose 9 (where order does matter) yields 1,816,214,400 unique combinations