Help Basic Probabilities

[Reflection46n2]

I consider myself a relative beginner to texas hold'em as a college student but i am having trouble understanding how say with 47 unseen cards and 9 to help me (flush draw) how i will win 35% of the time. Is this done using random variables? (in)depenent events? Standard deviations? i cant seem to find a true explanation of this. Any info or links are greatly appreciated.