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Re: Practice questions of Combinations and probability [#permalink]
05 Apr 2013, 03:51

Thank you for the collection. As I worked through the questions i noticed that some answers are not correct. For example answer 5 should be "B) 66" and not D. However, explaination is correct in my opinion. Cheers,

Re: Practice questions of Combinations and probability [#permalink]
16 Apr 2014, 02:06

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Re: Practice questions of Combinations and probability [#permalink]
22 Sep 2014, 14:32

I have a doubt regarding Q4 in the doc: How many 3-digit numbers satisfy the following conditions: The first digit is different from zero and the other digits are all different from each other?

The answer in the doc is 648 but it is derived on the basis - 9 X 9 X 8.

The question does not say the second and third digits are different from the first one. It says they are different from each other. Then should it not be 9 X 10 X 9? Am I mistaken?

Re: Practice questions of Combinations and probability [#permalink]
22 Sep 2014, 23:23

Expert's post

archie9 wrote:

I have a doubt regarding Q4 in the doc: How many 3-digit numbers satisfy the following conditions: The first digit is different from zero and the other digits are all different from each other?

The answer in the doc is 648 but it is derived on the basis - 9 X 9 X 8.

The question does not say the second and third digits are different from the first one. It says they are different from each other. Then should it not be 9 X 10 X 9? Am I mistaken?

I see your point. Ambiguous wording. Ignore this question. _________________

Re: Practice questions of Combinations and probability [#permalink]
28 Nov 2014, 16:38

Since 1 appears exactly three times, we can solve for the other four digits only. For every digit we can choose out of 8 digits only (without 1 and 0). Since we have 4 prime digits (2, 3, 5, 7) and 4 non-prime digits (4, 6, 8, 9), the probability of choosing a prime digit is ½. We need at least two prime digits: One minus (the probability of having no prime digits + having one prime digit): There are 4 options of one prime digit, each with a probability of (1/2)4. There is only one option of no prime digit with a probability of (1/2)4. So: [1- ((1/2)4+(1/2)4*4)] = 11/16. – Bernoulli’s principle

Hi Narenn This is in regards to your probability question bank 25 .

I dint quite get this - if there are three spots to be filled with 2 prime numbers - would 1/2(probablity of picking the first prime) * 3/7(probablity of picking the second one) would that not be enough - or are we assuming that there can be the two prime number which are same??

Would really appreciate your response

gmatclubot

Re: Practice questions of Combinations and probability
[#permalink]
28 Nov 2014, 16:38

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