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# A is given a phone list containing 200 different phone

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CEO
Joined: 15 Aug 2003
Posts: 3454
A is given a phone list containing 200 different phone  [#permalink]

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06 Dec 2003, 17:26
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24.A is given a phone list containing 200 different phone
numbers. B is given the same phone list A and B are ask to call one phone
number at the same time.

a. What is the probability that A and B call the same number?

b. What is the probability that A and B call a specific number?

Is a different from b.
Director
Joined: 28 Oct 2003
Posts: 501
Location: 55405

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06 Dec 2003, 21:32
Quote:
a. What is the probability that A and B call the same number?

1/200.

Quote:
b. What is the probability that A and B call a specific number?

the probability that A calls a specific number:1/200
the probability that B calls the same specific number: 1/200

1/40000 is the probability of both A and B calling the same, prespecified.

This might be confusing, but try to look at it this way, and assume the numbers are simply 1,2,3,4...200

A could call any of these numbers, so there's a 1/200 chance of A calling a specific number, say, #153
B could call any of these numbers, so there's a 1/200 chance of B calling a specific number, say, #153
1/40000 is the chance of A and B both calling #153.

but, the odds of A and B calling the same number (any number) is still 1/200.

I can try again if this makes no sense.
GMAT Instructor
Joined: 07 Jul 2003
Posts: 770
Location: New York NY 10024
Schools: Haas, MFE; Anderson, MBA; USC, MSEE

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07 Dec 2003, 05:50
stoolfi wrote:
Quote:
a. What is the probability that A and B call the same number?

1/200.

Quote:
b. What is the probability that A and B call a specific number?

the probability that A calls a specific number:1/200
the probability that B calls the same specific number: 1/200

1/40000 is the probability of both A and B calling the same, prespecified.

This might be confusing, but try to look at it this way, and assume the numbers are simply 1,2,3,4...200

A could call any of these numbers, so there's a 1/200 chance of A calling a specific number, say, #153
B could call any of these numbers, so there's a 1/200 chance of B calling a specific number, say, #153
1/40000 is the chance of A and B both calling #153.

but, the odds of A and B calling the same number (any number) is still 1/200.

I can try again if this makes no sense.

The easiest way to explain 1 is to say:

No matter what number A calls, there is a 1/200 chance that B will match it. Hence, the prob is 1/200.
_________________

Best,

AkamaiBrah
Former Senior Instructor, Manhattan GMAT and VeritasPrep
Vice President, Midtown NYC Investment Bank, Structured Finance IT
MFE, Haas School of Business, UC Berkeley, Class of 2005
MBA, Anderson School of Management, UCLA, Class of 1993

CEO
Joined: 15 Aug 2003
Posts: 3454

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07 Dec 2003, 07:08
stoolfi wrote:
Quote:
a. What is the probability that A and B call the same number?

1/200.

Quote:
b. What is the probability that A and B call a specific number?

the probability that A calls a specific number:1/200
the probability that B calls the same specific number: 1/200

1/40000 is the probability of both A and B calling the same, prespecified.

This might be confusing, but try to look at it this way, and assume the numbers are simply 1,2,3,4...200

A could call any of these numbers, so there's a 1/200 chance of A calling a specific number, say, #153
B could call any of these numbers, so there's a 1/200 chance of B calling a specific number, say, #153
1/40000 is the chance of A and B both calling #153.

but, the odds of A and B calling the same number (any number) is still 1/200.

I can try again if this makes no sense.

perfect.

for question 1
A could pick any #....we tell A to pick a number. Then ask B to go pick the same number. that would be 1/200 prob.

for question 2
We give both of them a number ..say 153..and go ask them to pick that number... then prob that A picks it is 1/200 and B picks it is 1/200

thanks stoolfi
praetorian
07 Dec 2003, 07:08
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