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Updated on: 13 Jul 2015, 00:48
Originally posted by crackgmat750 on 19 Jul 2004, 21:38.
Last edited by Bunuel on 13 Jul 2015, 00:48, edited 1 time in total.
Last edited by Bunuel on 13 Jul 2015, 00:48, edited 1 time in total.
Renamed the topic, edited the question and added the OA.
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A
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
95%
(hard)
Question Stats:
39% (02:34) correct
61%
(02:37)
wrong
based on 461
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A bowl is filled with consecutively numbered tiles from 1 to x. Joe pulls out a tile and uses it to construct Sequence Q, which consists of 10 consecutive integers starting with the number drawn. If Joe then selects one number from Sequence Q, what is the probability that the selected number is a multiple of 3?
(1) The last number in Sequence Q is a prime number that is less than 20.
(2) x <= 10
(1) The last number in Sequence Q is a prime number that is less than 20.
(2) x <= 10
19 Jul 2004, 22:55
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My answer is A.
We're looking for the number of multiples of three in a list of 10 consecutive numbers. If the list starts with a multiple of three, it will end with a multiple of three, and there will be 4 multiples in the list, making the probability 4/10. If it starts with any other number, the list will have exactly 3 multiples of 3, and the probability will be 3/10. Try a few and you'll see it.
So the question is just to figure out what it starts and ends with.
1) says that the last number is a prime number less than 20. That means it's not a multiple of 3, so there can't be 4 multiples, but there must be 3 multiples. --> sufficient.
2) says the first number is less than or equal to 10. Since that can be anything, multiple of 3 or not, we cannot know --> Insufficient.
We're looking for the number of multiples of three in a list of 10 consecutive numbers. If the list starts with a multiple of three, it will end with a multiple of three, and there will be 4 multiples in the list, making the probability 4/10. If it starts with any other number, the list will have exactly 3 multiples of 3, and the probability will be 3/10. Try a few and you'll see it.
So the question is just to figure out what it starts and ends with.
1) says that the last number is a prime number less than 20. That means it's not a multiple of 3, so there can't be 4 multiples, but there must be 3 multiples. --> sufficient.
2) says the first number is less than or equal to 10. Since that can be anything, multiple of 3 or not, we cannot know --> Insufficient.
General Discussion
19 Jul 2004, 22:19
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I think it is B.
(1)If Q is less than 20 and a prime number too, then Q can be 19,17,13,11 etc. This gives us different probabilities because of multiples of 3 vary depending on the last prime number.
(2) If x <= 10, then because the sequence consists of 10 integers, it should be between 1 and 10. It has 3,6,9 as multiples of 3. The probability is C(3,1) / C(10,1) = 3/10
What is OA?
(1)If Q is less than 20 and a prime number too, then Q can be 19,17,13,11 etc. This gives us different probabilities because of multiples of 3 vary depending on the last prime number.
(2) If x <= 10, then because the sequence consists of 10 integers, it should be between 1 and 10. It has 3,6,9 as multiples of 3. The probability is C(3,1) / C(10,1) = 3/10
What is OA?
crackgmat750
