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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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 )

Adding all these,

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

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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