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More ways to get Braingle...

Telephoney

Probability puzzles require you to weigh all the possibilities and pick the most likely outcome.

 

Puzzle ID:#10817
Fun:*** (2.19)
Difficulty:*** (2.28)
Category:Probability
Submitted By:mad-adeAhu***
Corrected By:Winner4600

 

 

 



One boring, rainy Madadian evening, Mad Ade sat waiting for the "Sweaty Chef Kebab" shop to open. He was staring aimlessly at the telephone. He began to wonder to himself, "If you were to dial any 7 digits on a telephone in random order, what is the probability that you will dial your own phone number?"

Assume that your telephone number is 7 digits.




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Comments

jimbo*au*
Mar 12, 2003

I think the answer according to the wording is 1 in 10 000 000. The teaser said dial in random order. Like if a monkey started pushing the buttons. The monkey is not going to say hey I had better not push zero first up because that's not a proper phone number!
mad-adeAhu*
Mar 12, 2003

the answer does say it is 1/10,000,000 if the first number is a zero, and 1/9,000,000 if you only count possible valid phone numbers.
dazza
Mar 12, 2003

A small technicality for Australian viewers. Here in Oz we have 8 digit phone numbers. Dialing 7 random numbers has 0% probability of getting anyone's phone number at all, least of all your own!!!
mad-adeAhu*
Mar 13, 2003

and that is why the teaser clearly states "assume that your telephone number is 7 digits"
Pheonix_down*
Apr 10, 2003

Easy to figure out but harder to explain why....
gogogo1
May 23, 2003

If the number could have started with a zero, it would have been 10 to the power of 7 because there would have been 10 digits possible.

Thats why the answer is 100,000,000
beanie89**
Aug 11, 2003

Who really cares?
Varthen**
Jul 02, 2004

thats wrong because 0 MUST be inclided if the number is random
piett
Aug 01, 2006

In the US at least, the 1st number cannot be a 1 or 0 (1 calls long-distance, 0 calls the operator), and the fourth number cannot be 9, as this is reserved for pay phones. There may be other restrictions, but I doubt it.
mathgrantAus*
Nov 18, 2008

This isn't about how many possible valid telephone numbers there are; this is about the probability that seven random digits will match a specified phone number. All 10,000,000 of the possible 7-digit numbers are equally likely to result from the randomization process, be they valid phone numbers or not. The given answer is wrong.



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