P (3,2) means that we are choosing any 2 things out of 3 and then permuting (giving all the possible orders of the things selected )
So...suppose the math books are A, B and C
The possible selections we can make is
{ A, B} { B , C} or { A, C} (1)
But we can also arrange these as { B, A} {C, B} or {C, A}.......so we have a total of 6 possible arrangements
Note that (1) is C(3,2) = 3 = the number of possible sets
But P(3,2) = 6 = the number of posssible arrangements of these sets
[ This stuff is hard for everyone !!! ]
