8 = 3 blue balls + 15 / 3
There are 18 balls in a jar. You take out 3 black balls without putting them back inside, and now the probability of pulling out a black ball is 1/3. How many blue balls were there in the beginning?
(a) 12.
(b) 9.
(c) 8.
(d) 7.
(e) 6.