Which expression represents the number of possible combinations of n items taken rat a time

nCr =  n! / (r!)(n-r!).

Step-by-step explanation:

That would be nCr =  n! / (r!)(n-r!).

A quick way of working these out is as follows:

Take 7C4:

this  is  7*6*5*4/4*3*2*1   Cancelling out the 4 and 3*2:

=  7*5

= 35.

15C12 = 15C(15-12)

= 15C3

= 15*14*13 / 3*2*1

=  5*7*13

= 455.

19

step-by-step explanation: