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by ssgmatter » Sat Jun 12, 2010 12:59 am
A and B are in a line to purchase tickets. How many people are in the line?
(1) There are 15 people behind A and 15 people in front of B.
(2) There are 5 people between A and B.
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Source: — Data Sufficiency |
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by ankurmit » Sat Jun 12, 2010 1:12 am
ans should be C
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by ssgmatter » Sat Jun 12, 2010 1:20 am
ankurmit wrote:ans should be C
OA is E
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by kvcpk » Sat Jun 12, 2010 1:32 am
Good Question!!

Answer is E. In the first thought answer seems to be C. But the point to remeber here is that 15 people behind A can contain B also. similarly 15 people infont of B may conatin A also.

Hence the answer is E.

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by ssgmatter » Sat Jun 12, 2010 1:34 am
kvcpk wrote:Good Question!!

Answer is E. In the first thought answer seems to be C. But the point to remeber here is that 15 people behind A can contain B also. similarly 15 people infont of B may conatin A also.

Hence the answer is E.

Praveen
Can you please elaborate more on this?.....Explain in more details.....
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by kvcpk » Sat Jun 12, 2010 1:43 am
ssgmatter wrote:
kvcpk wrote:Good Question!!

Answer is E. In the first thought answer seems to be C. But the point to remeber here is that 15 people behind A can contain B also. similarly 15 people infont of B may conatin A also.

Hence the answer is E.

Praveen
Can you please elaborate more on this?.....Explain in more details.....
Assume the sequence is in this order...
(15 Front of B)B(Five in between)A(Fifteen behind A)
This gives us the total number of people to be 37.

Now see this pattern
(9 in front of B)A(5 in front of B same as 5 between A and B)B(9 behind A)

This also satisfies both the premises. But the total this time is only 25.

Hence we cannot say for sure that total number of people. hence E.

Hope this time its clear. let m eknow if you have any queries.

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by ssgmatter » Sat Jun 12, 2010 1:44 am
kvcpk wrote:
ssgmatter wrote:
kvcpk wrote:Good Question!!

Answer is E. In the first thought answer seems to be C. But the point to remeber here is that 15 people behind A can contain B also. similarly 15 people infont of B may conatin A also.

Hence the answer is E.

Praveen
Can you please elaborate more on this?.....Explain in more details.....
Assume the sequence is in this order...
(15 Front of B)B(Five in between)A(Fifteen behind A)
This gives us the total number of people to be 37.

Now see this pattern
(9 in front of B)A(5 in front of B same as 5 between A and B)B(9 behind A)

This also satisfies both the premises. But the total this time is only 25.

Hence we cannot say for sure that total number of people. hence E.

Hope this time its clear. let m eknow if you have any queries.

Praveen
Please explain why B is insufficient?
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by ankurmit » Sat Jun 12, 2010 1:49 am
yes you are right.
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by kvcpk » Sat Jun 12, 2010 1:54 am
ssgmatter wrote: Please explain why B is insufficient?
B says there are 5 people in between A and B. But there is no information of how many people are before/behind A, B.
(people before A)A(5 people)B(People Behind A)
(people before B)B(5 people)A(People Behind B)

These are two of the possible scenarios. So you cannot tell for sure how many people on the whole are present in the line.
We only have information about 5 people in between A nad B.

Clear this time?

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by GMATGuruNY » Mon Jun 14, 2010 8:53 pm
With DS problems, I always ask myself the following three questions:

What do I want?
What do I have?
What do I need?

With DS problems, the goal is not to solve but to determine whether the statement gives you sufficient information to solve. In other words: Is the statement giving me what I need?

In this case:

What do I want? What is the question asking for? In this case, the number people in line.
What do I have? Before I look at the two statements, what information have I been given? In this case, that A and B are in the line.
What do I need? In this case, I'm probably looking for more info about where in the line A and B are standing.

Statement 1: We're told that there are 15 people behind A and 15 people in front of B, but no info is given about the number in between A and B, so there's no way to determine the total number of people in line. INSUFFICIENT.

Statement 2: We're told that there are 5 people between A and B, but no info is given about how many are behind A and in front of B, so there's no way to determine the total number of people in line. INSUFFICIENT.

Statements 1 and 2 together:

We now know the following --

There are 15 people behind A and 15 people in front of B.
There are 5 people between A and B.

What we don't know is who comes first, A or B.

If A comes first, then the line would look like this: [15 people] A [5 people] B [15 people], giving us 15 + A + 5 + B + 15 = 37 people in line.

If B comes first, then the line would look like this: [9 people ] B [5 people] A [9 people]. This scenario works because behind A are 9 + B + 5 = 15 people, in front of B are 5 + A + 9 = 15 people, and between A and B are 5 people. But now the total number of people in line is 9 + B + 5 + A + 9 = 25 people.

So even putting together the two statements, we can't determine the number of people in line.

The correct answer is E.
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by Testluv » Mon Jun 14, 2010 11:18 pm
Just some takeaways for number line problems:

always think about distance AND order. Most quasi-number line problems in DS will turn on not assuming the easy order.

Also, draw it out on your scratchpaper.
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