GMAT Question of the Day - Daily to your Mailbox; hard ones only

 It is currently 18 Jul 2018, 02:03

### GMAT Club Daily Prep

#### 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

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

# gmatclub test question : mary and john

Author Message
Manager
Joined: 06 Jul 2007
Posts: 249
gmatclub test question : mary and john [#permalink]

### Show Tags

18 Mar 2009, 13:10
The basket contains 3 white and 5 blue balls. Mary will extract one ball at random and keep it. After that, John will extract one ball at random. What is the probability that John will extract a blue ball?

1) 1/3

2) 3/8

3) 1/2

4) 5/8

5) 11/16
SVP
Joined: 29 Aug 2007
Posts: 2425
Re: gmatclub test question : mary and john [#permalink]

### Show Tags

18 Mar 2009, 20:24
sanjay_gmat wrote:
The basket contains 3 white and 5 blue balls. Mary will extract one ball at random and keep it. After that, John will extract one ball at random. What is the probability that John will extract a blue ball?

1) 1/3
2) 3/8
3) 1/2
4) 5/8
5) 11/16

= (3/8) (5/7) + (5/8) (4/7)
= 35/56
= 5/8
_________________

Gmat: http://gmatclub.com/forum/everything-you-need-to-prepare-for-the-gmat-revised-77983.html

GT

Manager
Joined: 06 Jul 2007
Posts: 249
Re: gmatclub test question : mary and john [#permalink]

### Show Tags

19 Mar 2009, 00:36
GMAT TIGER wrote:
sanjay_gmat wrote:
The basket contains 3 white and 5 blue balls. Mary will extract one ball at random and keep it. After that, John will extract one ball at random. What is the probability that John will extract a blue ball?

1) 1/3
2) 3/8
3) 1/2
4) 5/8
5) 11/16

= (3/8) (5/7) + (5/8) (4/7)
= 35/56
= 5/8

Well, once Mary has picked up a ball, it should not have any affect on the probability of John's extracting a blue ball. Isn't it?
Director
Joined: 01 Apr 2008
Posts: 827
Name: Ronak Amin
Schools: IIM Lucknow (IPMX) - Class of 2014
Re: gmatclub test question : mary and john [#permalink]

### Show Tags

26 Mar 2009, 06:34
Hi Gmat tiger,
why do we add these two probabilites and not multiply?

GMAT TIGER wrote:
sanjay_gmat wrote:
The basket contains 3 white and 5 blue balls. Mary will extract one ball at random and keep it. After that, John will extract one ball at random. What is the probability that John will extract a blue ball?

1) 1/3
2) 3/8
3) 1/2
4) 5/8
5) 11/16

= (3/8) (5/7) + (5/8) (4/7)
= 35/56
= 5/8
CIO
Joined: 02 Oct 2007
Posts: 1207
Re: gmatclub test question : mary and john [#permalink]

### Show Tags

30 Mar 2009, 05:46
We add them because these two probabilities represent two scenarios, either of which will satisfy the condition.

The first one is that Mary will pick a white ball $$\left(P=\frac{3}{8}\right)$$ and then John will pick a blue one with the probability of $$\frac{5}{7}$$ (note that Mary keeps her white ball and John picks his from the 7 left, not 8).

The second scenario is that Mary picks a blue ball $$\left(P=\frac{5}{8}\right)$$ and John picks a blue ball with a probability of $$\frac{4}{7}$$.

We have to add the probabilities of both scenarios because either of them is good for us:

$$\frac{3}{8}*\frac{5}{7} + \frac{5}{8}*\frac{4}{7} = \frac{35}{56} = \frac{5}{8}$$

Hope this helps.
Economist wrote:
Hi Gmat tiger,
why do we add these two probabilites and not multiply?

GMAT TIGER wrote:
sanjay_gmat wrote:
The basket contains 3 white and 5 blue balls. Mary will extract one ball at random and keep it. After that, John will extract one ball at random. What is the probability that John will extract a blue ball?

1) 1/3
2) 3/8
3) 1/2
4) 5/8
5) 11/16

= (3/8) (5/7) + (5/8) (4/7)
= 35/56
= 5/8

_________________

Welcome to GMAT Club!

Want to solve GMAT questions on the go? GMAT Club iPhone app will help.
Result correlation between real GMAT and GMAT Club Tests
Are GMAT Club Test sets ordered in any way?

Take 15 free tests with questions from GMAT Club, Knewton, Manhattan GMAT, and Veritas.

GMAT Club Premium Membership - big benefits and savings

SVP
Joined: 29 Aug 2007
Posts: 2425
Re: gmatclub test question : mary and john [#permalink]

### Show Tags

31 Mar 2009, 21:16
Sorry guys I didnot review this post earlier.

Same explanation above/under:

dzyubam wrote:
We add them because these two probabilities represent two scenarios, either of which will satisfy the condition.

The first one is that Mary will pick a white ball $$\left(P=\frac{3}{8}\right)$$ and then John will pick a blue one with the probability of $$\frac{5}{7}$$ (note that Mary keeps her white ball and John picks his from the 7 left, not 8).

The second scenario is that Mary picks a blue ball $$\left(P=\frac{5}{8}\right)$$ and John picks a blue ball with a probability of $$\frac{4}{7}$$.

We have to add the probabilities of both scenarios because either of them is good for us:

$$\frac{3}{8}*\frac{5}{7} + \frac{5}{8}*\frac{4}{7} = \frac{35}{56} = \frac{5}{8}$$

Hope this helps.
Economist wrote:
Hi Gmat tiger,
why do we add these two probabilites and not multiply?

_________________

Gmat: http://gmatclub.com/forum/everything-you-need-to-prepare-for-the-gmat-revised-77983.html

GT

Re: gmatclub test question : mary and john   [#permalink] 31 Mar 2009, 21:16
Display posts from previous: Sort by

# gmatclub test question : mary and john

Moderator: Bunuel

# Events & Promotions

 Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.