A bag contains 3 red balls, 4 green balls, and 5 yellow balls. If balls are drawn one at a time without replacement, what is the probability that the first yellow ball is drawn on the eighth draw? Express your answer as a common fraction.
Here's my take
We would have to draw all the red and green balls in the first 7 draws
The probability of this is (7/12) (6/11) (5/10) ( 4/9) ( 3/8) ( 2/ 7) ( 1/ 6) = 1/ 792
+201 
