It is currently 25 Jun 2017, 17:37

### 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

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# Probability question

Author Message
Intern
Joined: 04 Sep 2005
Posts: 8

### Show Tags

06 Sep 2005, 21:11
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

Could somebody please help me with this probability question--this was on the real GMAT...I'm not entirely sure of the numbers but the question is more or less the same.

There are three types of telephones:

telephone 1 - 65
telephone 2 - 75
telephone 3 - 110

total: 250

If two telephones are selected, what is the probability that both will be the same type of telephone?

(I'm reluctant to list the choices because I'm not sure if the numbers are correct)
Senior Manager
Joined: 27 Aug 2005
Posts: 331

### Show Tags

06 Sep 2005, 21:23
OK, here's my reasoning. I hope it's right:

The theory is that this is a pick without replacement question. The first pick is a free one; you have an 100% probability that the first telephone you pick will be one of the three types. The second one is the probability that, assuming you remove one of the first type, you'll pick one of the second type.

If the first one you pick is a Telephone 1, your chances of the second one being a Telephone 1 are 64/249.

If the first one you pick is a Telephone 2, your chances of the second one being a Telephone 2 are 74/249.

If the first one you pick is a Telephone 3, your chances of the second one being a Telephone 3 are 109/249.

But of course, each of those three scenarios has a different chance of happening. The first scenario has a 65/250 probability, the second scenario has a 75/250 probability and the third scenario has a 110/250 probability.

So the total probability = (65/250x64x249) + (75/250)(74/249) + (110/250)(109/249)

= 434/1245
Senior Manager
Joined: 15 Aug 2005
Posts: 257
Location: Las Vegas, NV

### Show Tags

06 Sep 2005, 21:47
coffeeloverfreak wrote:
OK, here's my reasoning. I hope it's right:

The theory is that this is a pick without replacement question. The first pick is a free one; you have an 100% probability that the first telephone you pick will be one of the three types. The second one is the probability that, assuming you remove one of the first type, you'll pick one of the second type.

If the first one you pick is a Telephone 1, your chances of the second one being a Telephone 1 are 64/249.

If the first one you pick is a Telephone 2, your chances of the second one being a Telephone 2 are 74/249.

If the first one you pick is a Telephone 3, your chances of the second one being a Telephone 3 are 109/249.

But of course, each of those three scenarios has a different chance of happening. The first scenario has a 65/250 probability, the second scenario has a 75/250 probability and the third scenario has a 110/250 probability.

So the total probability = (65/250x64x249) + (75/250)(74/249) + (110/250)(109/249)

= 434/1245

Exactly what I did. Anyone else?
Senior Manager
Joined: 04 May 2005
Posts: 279
Location: CA, USA

### Show Tags

06 Sep 2005, 21:48
here is how I view it, the result is the same as previous post:

favorable outcome:
65C2 + 75C2 + 110C2

all outcome:
250C2

the probablity is:

(65x64/2 + 75x74/2 + 110x109/2)/(250x249/2)

Last edited by qpoo on 06 Sep 2005, 23:00, edited 1 time in total.
Intern
Joined: 04 Sep 2005
Posts: 8

### Show Tags

06 Sep 2005, 22:17
Thanks...that's what I thought. (after the fact)
If I remember correctly, the actual answer choices were clean decimals a) .15 b).20 c).30 d)0.35 e) .50

I'd be very impressed if anyone got the answer in less than two minutes.

jsun wrote:
Could somebody please help me with this probability question--this was on the real GMAT...I'm not entirely sure of the numbers but the question is more or less the same.

There are three types of telephones:

telephone 1 - 65
telephone 2 - 75
telephone 3 - 110

total: 250

If two telephones are selected, what is the probability that both will be the same type of telephone?

(I'm reluctant to list the choices because I'm not sure if the numbers are correct)
Re: Probability question   [#permalink] 06 Sep 2005, 22:17
Display posts from previous: Sort by