If you want 1 blue marble and 1 red marble, then the probability of 1 blue, followed by 1 red is (5/20)(6/19).
The probability of 1 red, followed by 1 blue is (6/20)(5/19).
So, the probability without respect to order is the sum of the 2 scenarios above: (5/20)*(6/19) + (6/20)*(5/19).
I hope this helps.
