[BruceZ]

In your equation P(OE) should actually be the number of ways to flop and open ender with two of the cards, not the probability. For 45x, the number of ways is 4*4*48 since there are 4 ways to get a 4, 4 ways to get a 5, and 48 ways to get the other card. Now this includes 458, so when we do the same thing for 58x, we shouldn't count this again, so we can't just multiply by 3. So for 58x the number of ways is 4*4*44 since we don't want 584. Now one of these ways includes 589, so when we do 89x we don't want 895, so we have again 4*4*44. So all together the probability is:

(4*4*48 + 4*4*44 + 4*4*44)/C(50,3) = 11.1%.