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# John wrote a phone number on a note that was later lost

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John wrote a phone number on a note that was later lost [#permalink]  24 May 2010, 14:15
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John wrote a phone number on a note that was later lost. John can remember that the number had 7 digits, the digit 1 appeared in the last three places and 0 did not appear at all. What is the probability that the phone number contains at least two prime digits?

A. 15/16
B. 11/16
C. 11/12
D. 1/2
E. 5/8
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Re: Phone number [#permalink]  25 May 2010, 03:46
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scaredshikless wrote:
25. John wrote a phone number on a note that was later lost. John can remember that the number had 7 digits, the digit 1 appeared in the last three places and 0 did not appear at all. What is the probability that the phone number contains at least two prime digits?

A. 15/16
B. 11/16
C. 11/12
D. 1/2
E. 5/8

I don't understand the explanation to this one (so I obviously got it wrong...).

Also, at first, I read it as 1 only appears once in the last three places. If that were the case, how would you solve this?

Thanks, folks.

The phone numbers is of a type: {X}{X}{X}{X}{1}{1}{1}.

{X}'s can take following values: 4 primes {2, 3, 5, 7} and 4 non-primes {4, 6, 8, 9}. Total 8 choices for each {X}. Probability that {X} will be prime is therefore \frac{4}{8}=\frac{1}{2} and probability of {X} will not be a prime is again \frac{1}{2}.

We want at least 2 {X}'s out of 4 to be primes, which means 2, 3 or 4 primes.

Let's count the opposite probability and subtract it from 1.

Opposite probability of at least 2 primes is 0 or 1 prime:

So {P}{NP}{NP}{NP} and {NP}{NP}{NP}{NP}.

Scenario 1 prime - {P}{NP}{NP}{NP}: \frac{4!}{3!}*\frac{1}{2}*(\frac{1}{2})^3=\frac{4}{16}. We are multiplying by \frac{4!}{3!} as scenario {P}{NP}{NP}{NP} can occur in several different ways: {P}{NP}{NP}{NP}, {NP}{P}{NP}{NP}, {NP}{NP}{P}{NP}, {NP}{NP}{NP}{P} - 4 ways (basically the # of permutations of 4 objects out ow which 3 are the same).

Scenario 0 prime - {NP}{NP}{NP}{NP}: (\frac{1}{2})^4=\frac{1}{16}.

Hence opposite probability = \frac{4}{16}+\frac{1}{16}=\frac{5}{16}.

So probability of at least 2 primes is: 1-(Opposite probability) = 1-\frac{5}{16}=\frac{11}{16}

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Re: Phone number [#permalink]  10 Sep 2012, 13:35
Bunuel wrote:
scaredshikless wrote:
25. John wrote a phone number on a note that was later lost. John can remember that the number had 7 digits, the digit 1 appeared in the last three places and 0 did not appear at all. What is the probability that the phone number contains at least two prime digits?

A. 15/16
B. 11/16
C. 11/12
D. 1/2
E. 5/8

I don't understand the explanation to this one (so I obviously got it wrong...).

Also, at first, I read it as 1 only appears once in the last three places. If that were the case, how would you solve this?

Thanks, folks.

The phone numbers is of a type: {X}{X}{X}{X}{1}{1}{1}.

{X}'s can take following values: 4 primes {2, 3, 5, 7} and 4 non-primes {4, 6, 8, 9}. Total 8 choices for each {X}. Probability that {X} will be prime is therefore \frac{4}{8}=\frac{1}{2} and probability of {X} will not be a prime is again \frac{1}{2}.

We want at least 2 {X}'s out of 4 to be primes, which means 2, 3 or 4 primes.

Let's count the opposite probability and subtract it from 1.

Opposite probability of at least 2 primes is 0 or 1 prime:

So {P}{NP}{NP}{NP} and {NP}{NP}{NP}{NP}.

Scenario 1 prime - {P}{NP}{NP}{NP}: \frac{4!}{3!}*\frac{1}{2}*(\frac{1}{2})^3=\frac{4}{16}. We are multiplying by \frac{4!}{3!} as scenario {P}{NP}{NP}{NP} can occur in several different ways: {P}{NP}{NP}{NP}, {NP}{P}{NP}{NP}, {NP}{NP}{P}{NP}, {NP}{NP}{NP}{P} - 4 ways (basically the # of permutations of 4 objects out ow which 3 are the same).

Scenario 0 prime - {NP}{NP}{NP}{NP}: (\frac{1}{2})^4=\frac{1}{16}.

Hence opposite probability = \frac{4}{16}+\frac{1}{16}=\frac{5}{16}.

So probability of at least 2 primes is: 1-(Opposite probability) = 1-\frac{5}{16}=\frac{11}{16}

Just a small confusion
2,3,5,7 are primes , and 1,4,6,8,9 non primes ,just because there are 3 one's in the last three places it doesn't mean that 1 cannot be at any other place, question says boy remembers last three places having one's and not that there are only 3 ones in the 7 digit number
the phone number could be {1 2 3 6 1 1 1 } this has two primes and 5 non primes and 4 one's
or { 8 2 2 6 1 1 1 } this has 2 primes and all the primes are same

first going the long way p( 2 primes ) +p( 3 primes )+p(4 primes )

\frac{4}{9} * \frac{4}{9} * \frac{5}{9} *\frac{5}{9} *1*1*1* \frac{4!}{2!2!} ( exactly two primes )
Multiplying by \frac{4!}{2!2!} as we can permutate only the first 4 digits , the last are fixed (1,1,1)

\frac{4}{9} * \frac{4}{9}* \frac{4}{9}*\frac{5}{9}*1*1*1 *\frac{4!}{3!} ( exactly 3 primes )( 3 of a kind )

\frac{4}{9} *\frac{4}{9} *\frac{4}{9} *\frac{4}{9} * 1,1,1 ( exactly 4 primes )

Now for the case 2 primes, both the primes could be same or different then how does the notation \frac{4!}{2!2!} change , or does it remain the same ?

Similarly for the case of 3 primes the primes could be 2,2,2 all same or 2,3,5 all different then how does the notation \frac{4!}{3! } change or does it remain the same?

In this question we are taking primes as one kind and non primes as other kind , so it doesn't matter if the primes are all same or all different ? Is this statement correct?

1)Please could you show how to do this sum individual probability way as I have tried above ?

2)Also please consider the fact that 1 may have to be included as a non prime as the question does not explicitly state that there are only 3 ones in the phone number , he only remembers that the last three are ones.

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Re: John wrote a phone number on a note that was later lost [#permalink]  11 Sep 2012, 05:34
Hi

Looking for some assistance with this question , please .
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Re: John wrote a phone number on a note that was later lost [#permalink]  11 Sep 2012, 12:12
i too tried the long way and could not decide whether the order of two primes are important or not. bunuel a little help please
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Re: Phone number [#permalink]  11 Sep 2012, 22:05
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stne wrote:
Just a small confusion
2,3,5,7 are primes , and 1,4,6,8,9 non primes ,just because there are 3 one's in the last three places it doesn't mean that 1 cannot be at any other place, question says boy remembers last three places having one's and not that there are only 3 ones in the 7 digit number
the phone number could be {1 2 3 6 1 1 1 } this has two primes and 5 non primes and 4 one's
or { 8 2 2 6 1 1 1 } this has 2 primes and all the primes are same

first going the long way p( 2 primes ) +p( 3 primes )+p(4 primes )

\frac{4}{9} * \frac{4}{9} * \frac{5}{9} *\frac{5}{9} *1*1*1* \frac{4!}{2!2!} ( exactly two primes )
Multiplying by \frac{4!}{2!2!} as we can permutate only the first 4 digits , the last are fixed (1,1,1)

\frac{4}{9} * \frac{4}{9}* \frac{4}{9}*\frac{5}{9}*1*1*1 *\frac{4!}{3!} ( exactly 3 primes )( 3 of a kind )

\frac{4}{9} *\frac{4}{9} *\frac{4}{9} *\frac{4}{9} * 1,1,1 ( exactly 4 primes )

Now for the case 2 primes, both the primes could be same or different then how does the notation \frac{4!}{2!2!} change , or does it remain the same ?

Similarly for the case of 3 primes the primes could be 2,2,2 all same or 2,3,5 all different then how does the notation \frac{4!}{3! } change or does it remain the same?

In this question we are taking primes as one kind and non primes as other kind , so it doesn't matter if the primes are all same or all different ? Is this statement correct?

1)Please could you show how to do this sum individual probability way as I have tried above ?

2)Also please consider the fact that 1 may have to be included as a non prime as the question does not explicitly state that there are only 3 ones in the phone number , he only remembers that the last three are ones.

Yes, it does seem that the language of the question is not clear. When I read the question, I also assumed 4 prime digits and 5 non prime (including 1). After all, the question only says that 1 appears in the last 3 places (and hence, we can ignore the last 3 places). It doesn't say that 1 does not appear anywhere else. But it seems that it might also imply that 1 appears only in the last 3 places (looking at the options, that was their intention). Anyway, I am sure that if this question actually appears and they mean to say that 1 cannot be at any other place, they will definitely mention it.
Let's calculate the probability of different number of prime numbers:

0 primes
Probability = (5/9)*(5/9)*(5/9)*(5/9)

1 prime
Probability = (4/9)*(5/9)*(5/9)*(5/9)*4 (there are 4 positions for the prime)

2 primes
Probability = (4/9)*(4/9)*(5/9)*(5/9)*4C2 (select 2 of the 4 positions for the primes)

3 primes
Probability = (4/9)*(4/9)*(4/9)*(5/9)*4 (4 positions for the non prime)

4 primes
Probability = (4/9)*(4/9)*(4/9)*(4/9)

Probability of at least 2 primes = 1 - (Probability of 0 prime + Probability of 1 prime)
Probability of at least 2 primes = Probability of 2 primes + Probability of 3 primes + Probability of 4 primes

The calculations are painful so let's leave it here. As I said, their intention was probability of prime = 1/2, probability of composite = 1/2 which makes the calculations simple.

The 4 positions are different but you can have the same prime on one or more positions. When you say 4/9, you are including all cases (2, 3, 5, 7) so you don't need to account for them separately. When you say, (4/9)*(4/9)*(4/9)*(4/9), you are including all cases e.g. (2222, 2353, 3577, 2357 etc). All you need to do it separate out the primes and the non primes. That you do by arranging primes and non primes as NNPP or NPNP or PPNN etc (as we did above)
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Save $100 on Veritas Prep GMAT Courses And Admissions Consulting Enroll now. Pay later. Take advantage of Veritas Prep's flexible payment plan options. Veritas Prep Reviews Manager Joined: 27 May 2012 Posts: 153 Followers: 0 Kudos [?]: 22 [0], given: 62 Re: Phone number [#permalink] 12 Sep 2012, 08:27 VeritasPrepKarishma wrote: stne wrote: Just a small confusion 2,3,5,7 are primes , and 1,4,6,8,9 non primes ,just because there are 3 one's in the last three places it doesn't mean that 1 cannot be at any other place, question says boy remembers last three places having one's and not that there are only 3 ones in the 7 digit number the phone number could be {1 2 3 6 1 1 1 } this has two primes and 5 non primes and 4 one's or { 8 2 2 6 1 1 1 } this has 2 primes and all the primes are same first going the long way p( 2 primes ) +p( 3 primes )+p(4 primes ) \frac{4}{9} * \frac{4}{9} * \frac{5}{9} *\frac{5}{9} *1*1*1* \frac{4!}{2!2!} ( exactly two primes ) Multiplying by \frac{4!}{2!2!} as we can permutate only the first 4 digits , the last are fixed (1,1,1) \frac{4}{9} * \frac{4}{9}* \frac{4}{9}*\frac{5}{9}*1*1*1 *\frac{4!}{3!} ( exactly 3 primes )( 3 of a kind ) \frac{4}{9} *\frac{4}{9} *\frac{4}{9} *\frac{4}{9} * 1,1,1 ( exactly 4 primes ) Now for the case 2 primes, both the primes could be same or different then how does the notation \frac{4!}{2!2!} change , or does it remain the same ? Similarly for the case of 3 primes the primes could be 2,2,2 all same or 2,3,5 all different then how does the notation \frac{4!}{3! } change or does it remain the same? In this question we are taking primes as one kind and non primes as other kind , so it doesn't matter if the primes are all same or all different ? Is this statement correct? 1)Please could you show how to do this sum individual probability way as I have tried above ? 2)Also please consider the fact that 1 may have to be included as a non prime as the question does not explicitly state that there are only 3 ones in the phone number , he only remembers that the last three are ones. Yes, it does seem that the language of the question is not clear. When I read the question, I also assumed 4 prime digits and 5 non prime (including 1). After all, the question only says that 1 appears in the last 3 places (and hence, we can ignore the last 3 places). It doesn't say that 1 does not appear anywhere else. But it seems that it might also imply that 1 appears only in the last 3 places (looking at the options, that was their intention). Anyway, I am sure that if this question actually appears and they mean to say that 1 cannot be at any other place, they will definitely mention it. Let's calculate the probability of different number of prime numbers: 0 primes Probability = (5/9)*(5/9)*(5/9)*(5/9) 1 prime Probability = (4/9)*(5/9)*(5/9)*(5/9)*4 (there are 4 positions for the prime) 2 primes Probability = (4/9)*(4/9)*(5/9)*(5/9)*4C2 (select 2 of the 4 positions for the primes) 3 primes Probability = (4/9)*(4/9)*(4/9)*(5/9)*4 (4 positions for the non prime) 4 primes Probability = (4/9)*(4/9)*(4/9)*(4/9) Probability of at least 2 primes = 1 - (Probability of 0 prime + Probability of 1 prime) Probability of at least 2 primes = Probability of 2 primes + Probability of 3 primes + Probability of 4 primes The calculations are painful so let's leave it here. As I said, their intention was probability of prime = 1/2, probability of composite = 1/2 which makes the calculations simple. The 4 positions are different but you can have the same prime on one or more positions. When you say 4/9, you are including all cases (2, 3, 5, 7) so you don't need to account for them separately. When you say, (4/9)*(4/9)*(4/9)*(4/9), you are including all cases e.g. (2222, 2353, 3577, 2357 etc). All you need to do it separate out the primes and the non primes. That you do by arranging primes and non primes as NNPP or NPNP or PPNN etc (as we did above) Thank you karishma, since there was no response for a while I thought my query was unreasonable, good to see at least some people appreciated what I wrote . Highly appreciate your response. Great so now I know probability of one prime 4/9* 5/9 * 5/9 *5/9 * 4= 2000/6561 PROBABILITY of 0 prime's = 5/9*5/9*5/9*5/9= 625/6561 probability of at least 2 primes = 1- ( 2000/6561 + 625/6561) 1-(2625/6561) = 3936/6561 = .59 now lets check by doing long way 2 primes 4/9 * 4/9 * 5/9*5/9* 4!/2!2! = 2400/6561 ( PPNN : two primes could be same ,2 , 2 or different 2, 3 doesn't matter we can think of this as, two of a kind and two of another kind ,hence 4!/2!2!) 3 primes 4/9 *4/9 *4/9*5/9*4 = 1280/6561 (4!/3! = 4) Three of one kind again PPPN. 4 primes 4/9*4/9*4/9*4/9 = 256/6561 all of the same kind , hence only one way to select them. PPPP total 3936/6561 = .59 Bingo! hence we can see both ways we are getting the same result. Karishma if there is any error in my understanding, please do point out, I have considered primes as one kind and non primes as another kind instead of the position logic which you have mentioned. _________________ - Stne Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 3780 Location: Pune, India Followers: 812 Kudos [?]: 3238 [1] , given: 138 Re: Phone number [#permalink] 12 Sep 2012, 19:42 1 This post received KUDOS Expert's post stne wrote: Great so now I know probability of one prime 4/9* 5/9 * 5/9 *5/9 * 4= 2000/6561 PROBABILITY of 0 prime's = 5/9*5/9*5/9*5/9= 625/6561 probability of at least 2 primes = 1- ( 2000/6561 + 625/6561) 1-(2625/6561) = 3936/6561 = .59 now lets check by doing long way 2 primes 4/9 * 4/9 * 5/9*5/9* 4!/2!2! = 2400/6561 ( PPNN : two primes could be same ,2 , 2 or different 2, 3 doesn't matter we can think of this as, two of a kind and two of another kind ,hence 4!/2!2!) 3 primes 4/9 *4/9 *4/9*5/9*4 = 1280/6561 (4!/3! = 4) Three of one kind again PPPN. 4 primes 4/9*4/9*4/9*4/9 = 256/6561 all of the same kind , hence only one way to select them. PPPP total 3936/6561 = .59 Bingo! hence we can see both ways we are getting the same result. Karishma if there is any error in my understanding, please do point out, I have considered primes as one kind and non primes as another kind instead of the position logic which you have mentioned. Everything seems to be in order. It doesn't matter whether you choose to think in terms of position or arrangement of Ps and Ns. The result would be the same. When I need all arrangements of PNNN, I can write it as 4!/3! (= 4) or I can say that I will select one place for the P in 4 different ways. Either ways, we are arranging a P and 3 Ns. _________________ Karishma Veritas Prep | GMAT Instructor My Blog Save$100 on Veritas Prep GMAT Courses And Admissions Consulting
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Re: John wrote a phone number on a note that was later lost [#permalink]  22 Aug 2013, 04:20
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Bumping for review and further discussion.
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Re: John wrote a phone number on a note that was later lost   [#permalink] 22 Aug 2013, 04:20
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