There are 21 balls in a jar. You take out 3 red balls without putting them back inside, and now the probability of pulling out a red ball is 1/3. How many blue balls were there in the beginning?  Choices
A) 12. B) 9. C) 8. D) 7. E) 6.
9= 3 blue balls + 18 / 3   Answer: 9.
<result>B</result>
There are 21 balls in a jar. You take out 3 red balls without putting them back inside, and now the probability of pulling out a red ball is 1/3. How many blue balls were there in the beginning?