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I use weight average formula to solve this question.

Formular : \([\frac{(A member)}{(all members)}*(average of A)]\) + \([\frac{(B member)}{(all members)}*(average of B)]\) = average of A and B

We know the number of students in class and assume that there is 1 teacher per class to make it easy to calculate.
So now we know :
Number of all member in class - 28+1
The ratio of student - \(\frac{ 28}{29 }\)
Ratio of teacher - \(\frac{1}{29}\)
Average age of student - 9
Average age of teacher - X
Average of Student and Teacher - 10

Apply the formula


\(\frac{28}{29}*(9) + \frac{1}{29}*(x)\) = 10
\(\frac{252}{29}+ \frac{x}{29}\) = 10
252+x = 290
x = 38

Answer C

This is my first time comments on solving question. Hope this comment is helpful!
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