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# Select 5 numbers from the series of 1 to 20 (inclusive).

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Senior Manager
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Select 5 numbers from the series of 1 to 20 (inclusive). [#permalink]

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12 Jul 2004, 07:25
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

Select 5 numbers from the series of 1 to 20 (inclusive). What is the probability that the selected number does not contain either 19 or 20.

Please explain procedure. (A simple one)
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12 Jul 2004, 07:48
Is it: 1-19C3/21C5.......!
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12 Jul 2004, 08:07
I think... it should be 18C5/20C5
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12 Jul 2004, 09:28
Agree with JPV
1-20 20 numbers - 5 can be selected in 20C5 ways
exluding 19,20 there are 18 numbers - 5 can be selected in 18C5 ways
Prob= 18C5/20C5
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12 Jul 2004, 11:07
For me, it's easier to do: 18/20x17/19x16/18x15/17x14/16, which cancels out very nicely and gives you 21/38.

(all this assuming there can't be repeated numbers)
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14 Jul 2004, 06:49
what if the same question had a little twist.....find the probability either 19 0r 20.....can any1 explain
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14 Jul 2004, 06:54
Hi
then it will be

Prob= 1-(18C5/20C5)

1-(21/38)

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14 Jul 2004, 07:01
carsen wrote:
Select 5 numbers from the series of 1 to 20 (inclusive). What is the probability that the selected number does not contain either 19 or 20.

Please explain procedure. (A simple one)

# of combinations of 5 numbers which doesn't contain either 19 or 20 is C[5,18].

# of possible combinations = C[5,20].

P(result) = C[5,18]/C[5,20] = (18!/13!5!)/(20!/15!5!) = 15*14/(20*19) = 3/4*14/19 = 42/76 = 21/38.
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14 Jul 2004, 07:08
thanks! Need some advice i have my gmat on monday and i been working only from kaplan, OG and once in a while from this forum.....do u think OG is reflective of the real quant questions and how close is powerprep tests to the real tests...any advice will be helpful
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14 Jul 2004, 07:28
sid_477 wrote:
thanks! Need some advice i have my gmat on monday and i been working only from kaplan, OG and once in a while from this forum.....do u think OG is reflective of the real quant questions and how close is powerprep tests to the real tests...any advice will be helpful

OG is very reflective of the real GMAT test, if you run a regression of actual GMAT scores on PP scores and Kaplan scores, you'll see that PP scores are more meaningful in this regression.

But there is of course one detail: OG and PP often do not contain hard problems, which appear on the test (deviation, mean, median, etc.; probability, combination, etc.). With this exception, OG and PP provide pretty real questions.
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14 Jul 2004, 07:51
I agree with emmanuel,

The OG is definately the source for real GMAT questions - anything else is someone's interpretation of them. But the hardest CAT questions aren't in the OG at all. I think the last 200 of the multiple choice and the last 100 of the data sufficiency are better than the first sets of questions - that's because they were the ones added for the 10th edition, so they're newer.

I take the GMAT every 6 months, so I speak from much experience.
14 Jul 2004, 07:51
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