The mean age group of some students is 15years. When the age of a teacher, 45 years old, is added to the ages of the students, the mean of their ages become 18 years. Find the number of students in the group.

The mean age group of some students is 15years. When the age of a teacher, 45 years old, is added to the ages of the students, the mean of their ages become 18 years. Find the number of students in the group.
A. 7
B. 9
C. 15
D. 42

Correct Option: Answer is B
Solution:

Mean (x)= (∑▒x)/N
15= (∑▒x)/N
15N = ∑▒x
When the age of a teacher 45 yrs is added,
18 =(15N+45)/(N+1) ; 18(N+1) =15N+45
=18N + 18 = 15N + 45; 18N-15N=45-18
3N=27; N = 9

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