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Say there are 47 cards left in the deck on the flop, and you have 10 outs to hit on the turn. That means that there are 10 cards that help you, and 37 cards that hurt you. The ratio of good cards to bad cards is 10:37, or 1:3.7. As a fraction, there are 10 out of 47 total cards that help you, which is 10/47, or 1 out of 4.7. It seems like you've got that part.
To see how it works for two streets, it's a little more complicated. The easiest way is to figure out the chance of missing both streets, then subtract that chance from 100%. The chance of missing on the turn is 37/47, and the chance of missing on the river is 36/46, so the chance of missing both is 37/47 * 36/46 = 0.6161 or 61.61%. That means the chance of hitting is 100% - 61.61% = 38.39%. So your chance of hitting is 38.39/100, or about 1/2.57.
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