Wrong Number
Thirteen percent of the people in San Diego have unlisted phone numbers. You select three hundred names as random from the phonebook. What is the expected number of people who have unlisted numbers?
Labels: logic, mathemagic, trickofmind
posted by Rajesh Lal at
6:19 AM
8 Comments:
I would expect that NONE of the people you phone will have unlisted numbers because the phone book is made up of the 87% of people who HAVE listed their numbers.
-Nevzat
yeah, none... if they're in the phonebook, there not unlisted.
after reading the question again, it could be taken in another way. The last statement say "What is the expected number of people who have unlisted numbers?" That would still be 13% of the population of LA. It did not ask the number within the 300 random selection.
My guess is zero
Zero because only listed numbers make it in the phonebook
(Same as above) and also zero because if they are in the phonebook they are in fact LISTED
not depending on San Diego or LA ... may be it is meant that listing 300 numbers and having 13% "between the lines" is 344.8... in total. That'd mean you have about 44 (rounded 45) numbers which are not listed.
Ha...funny. 0
If picked RANDOMLY, I say 39. Based on that 87% of phone numbers on the phone book are LISTED, then 87% of 300 of LISTED numbers would net 261 leaving with 39 UNLISTED numbers.
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