# Probability - Conditional Probability

#### AsianMan

There are 15 tennis balls in a box, of which 9 have NOT previously been used. Three of the balls are randomly chose, played with, and then RETURNED TO THE BOX. Later, another 3 balls are randomly chosen from the box. Find the probability that none of these balls has ever been used.

Seems pretty simple, right?

Attempted Solution:

Event A - 1st round balls have never been used before
Event B - 2nd round balls have never been used before

We want the probability of events A and B both occuring.

Prob(A and B) = Prob(B given A) * Prob(A)

Prob(A) = (9C3)/(15C3) = 84/455

Prob(B given A) = (6C3)/(15C3) = 20/455

Prob(A and B) = .008114962

Book says answer is .0893

EDIT: I got it now. I misunderstood the question.

Last edited:
Related Calculus and Beyond Homework News on Phys.org

### Want to reply to this thread?

"Probability - Conditional Probability"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving