Fifty percent of all the students attending a school on a : GMAT Data Sufficiency (DS)
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Fifty percent of all the students attending a school on a

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Fifty percent of all the students attending a school on a [#permalink]

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26 Feb 2013, 22:08
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Difficulty:

55% (hard)

Question Stats:

58% (02:15) correct 42% (01:12) wrong based on 100 sessions

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Fifty percent of all the students attending a school on a certain day arrived by 7:00 AM. How many students arrived by 7:00 AM on that day?

(1) Fifteen students arrived between 7:00 AM and 8:00 AM, and 4/5 of that day’s total attending students arrived by 8:00 AM.

(2) Ten students arrived after 8:00 AM that day.

Can anybody help me understand the question stem. The question is asking how many students arrived by 7:00am on that day but the first part of the question already says 50% of all the students arrived by 7am, so isn't the first part already answering the question itself. Seem confusing.

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Last edited by pikachu on 26 Feb 2013, 22:31, edited 1 time in total.
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Re: Fifty percent of all the students attending a school.... [#permalink]

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26 Feb 2013, 22:30
pikachu wrote:
Fifty percent of all the students attending a school on a certain day arrived by 7:00 AM. How many students arrived by 7:00 AM on that day?

(1) Fifteen students arrived between 7:00 AM and 8:00 AM, and 4/5 of that day’s total attending students arrived by 8:00 AM.

(2) Ten students arrived after 8:00 AM that day.

Can anybody help me understand the question stem. The question is asking how many students arrived by 7:00am on that day but the first part of the question already says 50% of all the students arrived by 7am, so isn't the first part already answering the question itself. Seem confusing.

You need to have a conclusive value for the number of students arriving by 7 am on that day. From the question stem, all we know is that 50% of the total students in attendance arrived by 7am.

From F.S.1, we know that 15 students arrived between 7 and 8 am. Also, by 8 am, total of 4/5 students had arrived. Thus, let the total number of students be 10x. Thus,

8x = 15+(the number of students who arrived by 7am) = 15+5x.

We can solve for x and get the value. Thus sufficient.

F.S 2 only states that 10 students arrived after 8 am. It tells nothing about the total number of students. There might be more students coming in. Thus insufficient.

A.
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Re: Fifty percent of all the students attending a school.... [#permalink]

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26 Feb 2013, 23:31
What do we know: 50 % of the students arrived before 7.

Statement (1): 80 % of the students arrived by 8 which means that 30 % of the students arrived between 7 and 8 which is equal to fifteen. From this you can calculate the total number of students. Then fifty percent of that will be the students who arrived before 7.
Hence Statement (1) is sufficient.

Statement (2) is insufficient because we just know how many students came after 8 and do not know anything about the number or percentage of students arrived between 7 and 8.
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Re: Fifty percent of all the students attending a school on a [#permalink]

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02 Apr 2013, 21:49
pikachu wrote:
Fifty percent of all the students attending a school on a certain day arrived by 7:00 AM. How many students arrived by 7:00 AM on that day?

(1) Fifteen students arrived between 7:00 AM and 8:00 AM, and 4/5 of that day’s total attending students arrived by 8:00 AM.

(2) Ten students arrived after 8:00 AM that day.

Can anybody help me understand the question stem. The question is asking how many students arrived by 7:00am on that day but the first part of the question already says 50% of all the students arrived by 7am, so isn't the first part already answering the question itself. Seem confusing.

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The question asks How Many? When the GMAT asks "How Many?" it needs a value.

So P = population of the school.

Given: .5P arrived by 7 am

Question is .5P = ?

1.) 15 students arrive between 7 and 8. $$\frac{4P}{5}$$ showed up by 8 am.

$$\frac{4P}{5} = .5P + 15$$

You can now solve for P. Sufficient.

2.) 10 students arrived after 8 am.

This tells you nothing about the students arriving between 7 am & 8 am or the total population of the school. Insufficient.

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Re: Fifty percent of all the students attending a school on a [#permalink]

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04 Feb 2014, 06:57
We have the following case for Statement 1
We need to find x

<7--x---7----8

--50%-- -15-
----80%-----

So it basically says that until 7am we have 50% of total and before 8am we have 80% of total
This means that those 15 students represent 30% of the total

Therefore we can find the total which are 50 students and 50%, 25 students arrived by 7am

Hence A is sufficient

From 'b' we can't solve the question

Hope it clarifies
Cheers
J
Re: Fifty percent of all the students attending a school on a   [#permalink] 04 Feb 2014, 06:57
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