David has shown you how to compute the odds of flopping 2-pair given two hole cards of different ranks using fractions rather than combinations. Still I'm not sure I could multiply these fractions to powers in my head at the table in a couple seconds. I could whip out a calculator, but that would be odd and probably cost me future action. So here's how I'd do the problem in my head using my 6% per board card approximation.


6% times 3 cards = 18% to catch one of the cards plus 6% times 2 other cards = 12% to catch the other card. I might not be able to multiply 12% times 18% so fast, so I'll just do 10% of 20% to give 2%. Hey that's the right answer!