The first choice can be any one of 6 letters. For each of those ...
The second choice can be any one of the remaining 5 .
Total numbers of ways to choose 2 letters is (6 x 5) = 30 .
BUT ... There are not 30 different results you can wind up with.
Let's say you ended up with the 'r' and the 's' in your hand.
That could have happened in two ways ... either you chose the 'r' first,
or you chose the 's' first.
EVERY pair of letters can be chosen in 2 different ways.
There are 30 different ways to choose a pair, but only 15 different pairs available.