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

Question

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)

cbrf3 · Answer

Is it: 1-19C3/21C5.......!

jpv · Answer

I think... it should be 18C5/20C5

srijay007 · Answer

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

ian7777 · Answer

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)

sid_477 · Answer

what if the same question had a little twist.....find the probability either 19 0r 20.....can any1 explain

carsen · Answer

Hi then it will be Prob= 1-(18C5/20C5) 1-(21/38)

Emmanuel · Answer

# of combinations of 5 numbers which doesn't contain either 19 or 20 is C .

# of possible combinations = C .

P(result) = C /C  = (18!/13!5!)/(20!/15!5!) = 15*14/(20*19) = 3/4*14/19 = 42/76 = 21/38.

sid_477 · Answer

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

Emmanuel · Answer

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.

ian7777 · Answer

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.

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

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