Originally Posted by Sang-drax
Solving this problem by expanding the cases isn't very good. It's easy to miss a few cases and when the problem gets more complex the method becomes really impractical.
The number of ways the three cards can be chosen: 50 C 3
This is the same as I posted before.
The number of combinations exactly one 5 and exactly one 10: 3 * 3 * (50-6)
(3 fives, 3 tens and the last card could be anything but one of those six cards)
Divide these two numbers and the resulting probability is 0.0202
This is the probability to get two-pair (not full house or trips) with the two different cards you hold on your hand after the flop (three cards).
The chances are creater because you can get two pairs in more ways, for example 6, 6, 5 in the flop and 5, 10 in your hand, but I think 2.02% answers Brain's question.