Which expression represents the number of possible combinations of n items taken rat a time? n!
(n-nin
n!
(n-
(n-1)!
n!
(n-1)!!
n!

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.  

answer: 21$

step-by-step explanation:

$42 / $14 = 3   (ratio 3: 1)

$7 x 3 = $21