Dpax2max
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#1
Report Thread starter 8 years ago
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This is actually an easy question, I'm just being stupid

The Probability that telephone calls to a railway timetable enquiry service are answered is 0.7

*the Probability of it not being answered is 0.3

Sam requires some information and decides that if his call is not answered he will call repeatedly until he obtains an answer


find the probability that to obtain an answer he has to call
1)exactly three times

2) at least three times




I have the answers but I have no idea how to obtain them
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Get me off the £\?%!^@ computer
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(Original post by Dpax2max)
This is actually an easy question, I'm just being stupid

The Probability that telephone calls to a railway timetable enquiry service are answered is 0.7

*the Probability of it not being answered is 0.3

Sam requires some information and decides that if his call is not answered he will call repeatedly until he obtains an answer


find the probability that to obtain an answer he has to call
1)exactly three times

2) at least three times




I have the answers but I have no idea how to obtain them
If he has to call exactly 3 times then you have

failure, failure, success

so the probability is ?????

At least 3 times is not less than three times which is 1- ?????
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ChrisQ
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It should be intuitive - a 0.3 chance the game continues after he tries calling once, and a 0.7 chance of it stopping as he gets a successful call - only consider what happens if it continues to the second stage.

Note that the game getting to the fourth stage is equivalent to him calling at least three times.

I hope that helps, I don't want to give away the answers!
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Dpax2max
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answer to 1) 0.063 2)0.09

source- textbook
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Dpax2max
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I still don't understand part2
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Get me off the £\?%!^@ computer
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(Original post by Dpax2max)
I still don't understand part2
P(at least 3)= 1 - P(less than three) = 1- [P(0) +P(1)] = ????
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