Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized for You

we will pick new questions that match your level based on your Timer History

Track Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:

Probability Dependent Events [#permalink]
23 Jun 2012, 03:48

Hi,

Need help to know the correct solution.

1 In a shipment of 20 computers, 3 are defective. Three computers are randomly selected and tested. What is the probability that all three are defective if the first and second ones are replaced after being tested?

Probability

3 X 3 X 3 = 27 20 20 20 8000

2 In a shipment of 20 computers, 3 are defective. Three computers are randomly selected and tested. What is the probability that all three are defective if the first and second ones are not replaced after being tested?

Probability

3 X 2 X 1 = 6 20 19 18 6840

Probability

3 In a shipment of 20 computers, 3 are defective. Three computers are randomly selected and tested. What is the probability that all three are NOT defective if the first and second ones are not replaced after being tested?

Probability

1 X 1 X 1 = 1 17 16 15 4080

4 In a shipment of 20 computers, 3 are defective. Three computers are randomly selected and tested. What is the probability that all three are NOT defective if the first and second ones are replaced after being tested?

Re: Probability Dependent Events [#permalink]
04 Jul 2012, 08:54

Hi Sanjiv,

It seems to me that numbers 1 & 2 are right on the money, but 3 & 4 are a little off.

You correctly setup numbers 1 & 2 with the probability of event 1 times the probability of event 2 etc. Question3 wants to know the probability of NOT picking defective computers. In the first pick there is a 17/20 chance of not picking a defective computer. In the second pick there is a 16/19 chance, and in the third pick there is a 15/18 chance, which equals 17/20 times 16/19 times 15/18. In question three it would be (17/20) (17/20) (17/20), or (17/20)^3.

From questions 1 and 2 it seems that you have a good grasp of how to do this type of question and that you just made a careless error on questions 3 and 4.

Finally, I don`t think that this is a dependent probability question. Each pick is totally independent of the other picks (although the probability has to be adjusted for how many objects are left). A dependent probability is when one event has some influence over another event occurring; for example, in pitching, where a pitcher is more likely to pitch a strike after having pitched a strike, in which case when calculating the chance of a strike we have to adjust for the probability of the previous pitch being a strike. To the best of my recollection I have never come across a dependent probability question on any GMAT prep test or on the GMAT itself.

Anyways, I hope this helps and best of luck to you, David

gmatclubot

Re: Probability Dependent Events
[#permalink]
04 Jul 2012, 08:54

1. My favorite football team to infinity and beyond, the Dallas Cowboys, currently have the best record in the NFL and I’m literally riding on cloud 9 because...

I couldn’t help myself but stay impressed. young leader who can now basically speak Chinese and handle things alone (I’m Korean Canadian by the way, so...