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# This is a DS problem from GmatClub tests. What is the mean

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This is a DS problem from GmatClub tests. What is the mean [#permalink]  13 Sep 2009, 11:07
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This is a DS problem from GmatClub tests. What is the mean age of all students of the city's schools?

1. The average age of the students in each of the city's schools is 12
2. The number of students in each of the city's schools is 70

I do not agree with OA. Will post it later. Can anyone clarify this, please?
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Re: the mean age [#permalink]  13 Sep 2009, 11:18
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barakhaiev wrote:
This is a DS problem from GmatClub tests. What is the mean age of all students of the city's schools?

1. The average age of the students in each of the city's schools is 12
2. The number of students in each of the city's schools is 70

I do not agree with OA. Will post it later. Can anyone clarify this, please?

If the average at each school is the same, then the overall average will be equal to the average at each school, so S1 is sufficient and the answer should be A.
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Re: the mean age [#permalink]  13 Sep 2009, 11:24
1
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Mean age = Total Sum of ages of all the students in all the schools / Total number of students

1.) Sum of ages of students in one school / number of students in that school = 12
=> Sum of ages of students in one school = 12 * (number of students in that school )
Similarly,
Sum of ages of students in second school / number of students in that school = 12
=> Sum of ages of students in Second school = 12 * (number of students in that school )

Sum of ages of all students in all schools = 12 * (total number of students)
=> Mean age = 12
Sufficient
2.) only gives, number of students in each school. in sufficient

ans A
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Re: the mean age [#permalink]  13 Sep 2009, 11:40
Yeah, the answer is A. Thank you very much for explanations. But I thought this question can not be so easy. While reading choice 1 at first, I remembered that term "average" applies to mean, median and mode. And thus, I picked answer E, because of this uncertainty...going crazy with GMAT
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Re: the mean age [#permalink]  18 Sep 2011, 01:55
barakhaiev wrote:
This is a DS problem from GmatClub tests. What is the mean age of all students of the city's schools?

1. The average age of the students in each of the city's schools is 12
2. The number of students in each of the city's schools is 70

Let there be 2 schools A and B
To find. Mean age = sum of ages of students in A and B / sum of number of students in A and B
= age A + age B/ no. A + no. B

1. The average age of the students in each of the city's schools is 12.
We have two schools A and B.
For A.
Avg1 = sum of ages of student in school A/ no. of student in school A
12 = sum of ages of student in school A/ no. of student in school A

sum of ages of student in school A = 12 * no. of student in school A

Avg2=sum of ages of student in school B/ no. of student in school B
12 = sum of ages of student in school B/ no. of student in school B

sum of ages of student in school B = 12 * no. of student in school B

Mean Age=sum of ages of student in school A+sum of ages of student in school B /no. of student in school A+ no. of student in school B
= 12 * no. of student in school A + 12 * no. of student in school B/no. of student in school A+ no. of student in school B
=12(no. of student in school A + no. of student in school B)/no. of student in school A+ no. of student in school B
=12.

Sufficient.

2. The number of students in each of the city's schools is 70.
Mean Age=sum of ages of student in school A+sum of ages of student in school B /no. of student in school A+ no. of student in school B
= sum of ages of student in school A+sum of ages of student in school B /70+70
=sum of ages of student in school A+sum of ages of student in school B /140.

sum of ages of student in school A+sum of ages of student in school B --we do not know this.

Insufficient.

OA A.
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Re: the mean age [#permalink]  05 Oct 2011, 12:37
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A

1 is enough as if the average of all the schools are 12 means then mean is also 12
Re: the mean age   [#permalink] 05 Oct 2011, 12:37
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