Hi.

I have the following combinatoric problem (well it's actually a probability problem that need to be resolved using combinatorics):

There are pairs of shoes in the closet.

shoes are chosen from it randomly. ( )

find the probability to get exactly pairs.

so this is what I'm thinking:

first of all: (the number of possibilities to choose 2m shoes from 2n).

but I just can't decide how many possibilities are there to choose exactly pairs out of .

at first I thought it would be , but that's not the right answer...

any help would be greatly appretiated.

*edit

btw, acoording to what I found somwehre online, the answer is

I just couldn't figure out why...