SOLUTION: In how many ways can letters be chosen from {J,K,L,M,N,O,P,Q,R,S} assuming that the order of the choices doesn't matter and that repeats are not allowed?

There are 10 letters in the set, in all, and they all are distinct.


You can select first letter by 10 ways.

Then you can select the second letter by 9 ways among the 9 remaining letters.

Then you can select the second letter by 8 ways among the 8 remaining letters.


    . . .        and   so   on   . . . 


Then you can select the 9-th letter by 2 ways among the 2 remaining letters.

Then you can select the last, 10-th letter by only 1 way among the 1 remaining letter 
    (practically, you just have no selection choice at this step).



In all, there are 10! = 10*9*8*7* . . . *2*1 = 7257600 ways/selections.    ANSWER