Help required - 13 consecutive integers

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Help required - 13 consecutive integers

by ganeshrkamath » Tue Aug 13, 2013 11:58 pm
In a sequence of 13 consecutive integers, all of which are less than 100, there are exactly 3 multiples of 6. How many integers in the sequence are prime?

(1) Both of the multiples of 5 in the sequence are also multiples of either 2 or 3.

(2) Only one of the two multiples of 7 in the sequence is not also a multiple of 2 or 3.

[spoiler]OA C

But I get the answer as E.
I get 2 solutions :
6,7,8,9,10,11,12,13,14,15,16,17,18 has 4 primes and
42,43,44,45,46,47,48,49,50,51,52,53,54 has 3 primes

Original explanation:
This one is a group problem in disguise. First, there's a lot of information buried in the question. If there are exactly three multiples of 6 in the sequence, then the sequence must start with a multiple of 6. So, there will always be 7 multiples of 2 and 5 multiples of 3 in the sequence. Those are two of the groups in the group formula. The total is, of course, 13. We'll also need to subtract the numbers that are both multiples of 2 and 3 (aka the multiples of 6) from the total. That means that the statements need to tell us about the multiples of 5 and 7. Everything else in the sequence (neither in the group formula) will be prime. Statement (1) tells us that we don't need to count either of the multiples of 5 since they have already been counted but it tells us nothing about the multiples of 7 in the sequence. Hence, BCE. Statement (2) tells us to count one of the multiples of 7 but tells us nothing about the multiples of 5. Cross off B. Finally, when we put the statements together we know that 13 = 7(the multiples of 2) + 5 (the multiples of 3) + 2 (the multiples of 5) + 2 (the multiples of 7) − 3 (the multiples of 6) − 2 (the multiples of 5 and 2 or 3) − 1 (the multiples of 7 and 2 or 3) + Primes. Hence, there are 3 primes in the sequence. The correct answer is C.[/spoiler]
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by macattack » Wed Aug 14, 2013 2:49 am
Honestly I used brute force to solve that problem:

Case 1: 12-13-14-15-16-17-18-19-20-21-22-23-24: 4 primes
42-43-44-45-46-47-48-49-50-51-52-53-54: 3 primes
Both sequence verify statement one and several answers are possible--->insufficient

Case 2: 30-31-32-33-34-35-36-37-38-39-40-41-42: 3 primes
42-43-44-45-46-47-48-49-50-51-52-53-54: 4 primes
Both sequence verify statement two and several answers are possible--->insufficient

Statement 1 and 2 combined
Only 3 sequence verify both statements
42....54
66....78
84...96
All of these sequence have 3 primes
Hence OA is C
Knowing that this took me 10-15 mins to solve I would cry for help from an expert
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by ganeshrkamath » Wed Aug 14, 2013 3:08 am
macattack wrote:Honestly I used brute force to solve that problem:

Case 1: 12-13-14-15-16-17-18-19-20-21-22-23-24: 4 primes
42-43-44-45-46-47-48-49-50-51-52-53-54: 3 primes
Both sequence verify statement one and several answers are possible--->insufficient

Case 2: 30-31-32-33-34-35-36-37-38-39-40-41-42: 3 primes
42-43-44-45-46-47-48-49-50-51-52-53-54: 4 primes
Both sequence verify statement two and several answers are possible--->insufficient

Statement 1 and 2 combined
Only 3 sequence verify both statements
42....54
66....78
84...96
All of these sequence have 3 primes
Hence OA is C
Knowing that this took me 10-15 mins to solve I would cry for help from an expert
Sequence 6-7-8-9-10-11-12-13-14-15-16-17-18 also satisfies both statements, but this sequence has 4 primes.
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by LanaFirefox » Wed Aug 14, 2013 4:57 am
First I realized that it has to start with a multiple of 6, so I wrote out the sequence
6,7,8,9,10,11,12,13,14,15,16,17,18 and thought that I am looking for this sequence with a possible multiplier so 6x,7x,8x, 9x and so on. Find x and you know the sequence, so can calculate primes. Maximum x = 100/18= 5, so x can take values 1,2,3,4,5.

1) several x allow the 10 and 15 to be multiples of 2 or 3, because already in the basic form those fives have a multiplier 2 and 3, so insuff;
2) Only one 7 NOT to be multiple of two or three, x has to be 1 or 5 (that would make 7x not a multiple of 2 or 3), but we don't know which one, so insuff.
TOGETHER try both scenarios. X= 1 satisfies both 1) and 2) constraints, but, x=5 adds another multiple of 5 in the sequence (35) and 1) explicitly tells us that there are only 2 multiples of 5 in the sequence, so it leaves us with only one scenario x=1. So the answer is C.
(The sequence takes its basic form, there are 4 primes).

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by macattack » Thu Aug 15, 2013 2:44 am
ganeshrkamath wrote:
macattack wrote:Honestly I used brute force to solve that problem:

Case 1: 12-13-14-15-16-17-18-19-20-21-22-23-24: 4 primes
42-43-44-45-46-47-48-49-50-51-52-53-54: 3 primes
Both sequence verify statement one and several answers are possible--->insufficient

Case 2: 30-31-32-33-34-35-36-37-38-39-40-41-42: 3 primes
42-43-44-45-46-47-48-49-50-51-52-53-54: 4 primes
Both sequence verify statement two and several answers are possible--->insufficient

Statement 1 and 2 combined
Only 3 sequence verify both statements
42....54
66....78
84...96
All of these sequence have 3 primes
Hence OA is C
Knowing that this took me 10-15 mins to solve I would cry for help from an expert
Sequence 6-7-8-9-10-11-12-13-14-15-16-17-18 also satisfies both statements, but this sequence has 4 primes.
My bad!! we need Mitch, Brent, Matt and the guys to intervene!! its killing me not to know the method
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by Matt@VeritasPrep » Thu Aug 15, 2013 9:53 am
Hey, if you're going to shine the Matt-Signal, I'll come running! :D

The only way we can have three multiples of 6 is a string of 13 consecutive integers is if the string starts and ends with a multiple of 6. So we know that to start. I'm going to assume that the prompt means to say 13 consecutive POSITIVE integers as well (otherwise the answer is clearly E, as a string of 13 negatives would have no primes).

S1:

This tells us 5, 25, 35, 55, 85 and 95 can't be in the sequence. So we must have 6-18, 12-24, 36-48, 60-72, 66-78, or 72-84. We've got some conflicting #'s of primes here, so this is insufficient.

S2:

This tells us we can't have 14+21, 21+28, 56+63, or 63+70. Still insufficient.

S1 and S2:

We've got 6-18, 36-48, 66-78, or 72-84.

6-18 has primes 7, 11, 13, and 17
66-78 has primes 67, 71, and 73

So it's E and the OA is wrong.

Just to illustrate why each set is acceptable:

6 to 18 has three multiples of 6 (6, 12, 18). Its multiples of 5 are 10 and 15, both of which are multiples of either 2 or 3. Its multiples of 7 are 7 and 14, exactly one of which is not a multiple of 2 or 3.

66 to 78 has three multiples of 6 (66, 72, and 78). Its multiples of 5 are 70 and 75, both of which are multiples of either 2 or 3. Its multiples of 7 are 70 and 77, exactly one of which is not also a multiple of 2 or 3.

EDIT:

Looking at the OA explanation, it seems the problem is that it treats any multiples of 7 as being composite - but if 7 itself can be in the set, 7 is a prime.

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by macattack » Thu Aug 15, 2013 10:16 pm
Matt@VeritasPrep wrote:Hey, if you're going to shine the Matt-Signal, I'll come running! :D

The only way we can have three multiples of 6 is a string of 13 consecutive integers is if the string starts and ends with a multiple of 6. So we know that to start. I'm going to assume that the prompt means to say 13 consecutive POSITIVE integers as well (otherwise the answer is clearly E, as a string of 13 negatives would have no primes).

S1:

This tells us 5, 25, 35, 55, 85 and 95 can't be in the sequence. So we must have 6-18, 12-24, 36-48, 60-72, 66-78, or 72-84. We've got some conflicting #'s of primes here, so this is insufficient.

S2:

This tells us we can't have 14+21, 21+28, 56+63, or 63+70. Still insufficient.

S1 and S2:

We've got 6-18, 36-48, 66-78, or 72-84.

6-18 has primes 7, 11, 13, and 17
66-78 has primes 67, 71, and 73

So it's E and the OA is wrong.

Just to illustrate why each set is acceptable:

6 to 18 has three multiples of 6 (6, 12, 18). Its multiples of 5 are 10 and 15, both of which are multiples of either 2 or 3. Its multiples of 7 are 7 and 14, exactly one of which is not a multiple of 2 or 3.

66 to 78 has three multiples of 6 (66, 72, and 78). Its multiples of 5 are 70 and 75, both of which are multiples of either 2 or 3. Its multiples of 7 are 70 and 77, exactly one of which is not also a multiple of 2 or 3.

EDIT:

Looking at the OA explanation, it seems the problem is that it treats any multiples of 7 as being composite - but if 7 itself can be in the set, 7 is a prime.
Phew!!! Thanks Matt much appreciated
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