# Probability on balls

• Jan 6th 2011, 05:34 PM
Ka9
Probability on balls
Q1: A bag contains 5 red balls, 5 green balls, and 6 blue balls. If 3 balls are removed at random and no ball is returned to the bag after removal, what is the probability that all 3 balls are red? Answer should be in common fraction.

To this problem, I tried....
5/16 * 4/15 * 3/14 = 1/56
Is my approach correct.

Also, I am confused to solve the above problem in the following method?
16c3 * 15c2 * 14c1

Also wants to know, which practice is best to follow?

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Q2: A bag contains 3 red balls, 3 green balls, and 4 blue balls. If 2 balls are removed at random and each ball is returned to the bag after removal, what is the probability that all 2 balls will be red?

3/10 * 3/10 = 9/100
or
10c3 * 10c3
I am not sure about the second method?

Thanks,
• Jan 6th 2011, 05:40 PM
mr fantastic
Quote:

Originally Posted by Ka9
Q1: A bag contains 5 red balls, 5 green balls, and 6 blue balls. If 3 balls are removed at random and no ball is returned to the bag after removal, what is the probability that all 3 balls are red? Answer should be in common fraction.

To this problem, I tried....
5/16 * 4/15 * 3/14 = 1/56
Is my approach correct. Mr F says: Yes.

Also, I am confused to solve the above problem in the following method?
16c3 * 15c2 * 14c1 Mr F says: Why try doing it in a way that requires much more computation and time.

Also wants to know, which practice is best to follow? Mr F says: The way that you feel most comfortable with in getting the correct answer.

=================

Q2: A bag contains 3 red balls, 3 green balls, and 4 blue balls. If 2 balls are removed at random and each ball is returned to the bag after removal, what is the probability that all 2 balls will be red?

3/10 * 3/10 = 9/100 Mr F says: Correct.
or
10c3 * 10c3
I am not sure about the second method? Mr F says: Then do not complicate things by using this approach.

Thanks,

..
• Jan 6th 2011, 05:51 PM
Ka9
I want to know the second approach, if I get stuck with the first method, second will help me. Please tell me how to use the second approach.
• Jan 6th 2011, 05:56 PM
mr fantastic
Quote:

Originally Posted by Ka9
I want to know the second approach, if I get stuck with the first method, second will help me. Please tell me how to use the second approach.

Surely your textbook or class notes has examples of that approach that you can review ....?