An urn contains 5 red balls, 6 green balls, and 7 white balls. Three balls are drawn without replacement. What is the probability that the third ball is green given that the first two balls were white?
Since there's no replacement, once the white balls are taken out you only have 16 balls to choose from. After this I'm stumped on where to go.

There are initially 18 balls, and as you mentioned, once you have chosen and removed the first two (white) balls, you are left with 16 balls. 
Of those 16 balls, 6 are green. So the probability of picking a green ball is 6/16. 
