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The average age of $30$ students in a class is $12$ years. When the teacher's age is included, the average becomes $13$ years. Find the teacher's age.
A$35$
B$38$
C$43$
D$45$
Answer & Solution
Correct answer: C. $43$
Sum of $30$ students' ages $= 30 \times 12 = 360$. New sum (incl teacher) $= 31 \times 13 = 403$. Teacher's age $= 403 - 360 = 43$.
A common slip is to compute $30 \times (13 - 12) = 30$ and pick the wrong option — but you have to account for the teacher's own slot too.
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