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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