Gattaca,

Mooee is correct. The chance of hitting an open ended straight after the flop is indeed 31.5% and the chance of hitting a flush after the flop is 35%.

Lets assume you hold 8,9 and the flop reads 2,10,J.
You need a 7 or a Q to make you straight which is 8 outs (4 7's and a Q's)
The probability of NOT getting a 7 or a Q on the turn is 39/47 and then the probability of NOT getting a 7 or Q on the river is 38/46. if we multiply these together we get 1482/2162=0.6855 which is the probability that we DONT get a 7 or a Q on the turn and river. So the probability that we DO make our straight by catching 1 of 8 cards after the flop is 1-0.6855=0.3145=31.45%

For the flush draw we would need 1 of 9 cards on the turn or river. So the probability we dont make a flush is 38/47 x 37/46 = 1406/2162 = 0.6503. Therefore the chance of making a flush after having 4 out of 5 after the flop is 34.97%

Hope this helps

Cheers
ATC