6th June Learning summary of probability

Today I finished learning the series of  second method of permutations which is one with ordered sets and started with the third one of unordered sets of probabilities.

When you calculate a pair chosen from a group of 5, then you list all of possibilities which is 5 * 5, then minus repeated letters which is 5,

and then divided by 2, because there is a group of XY, then there will be an another group of YX in a pair.

So we can write it a  (N2-N)/2