The average age of 30 students is 9 years. If the age of their teacher is included, the average age becomes 10 years. The age of the teacher (in years) is :

A. 27 B. 31 C. 35 D. 40 Answer: Option D
Show Answer $\begin{array}{l}\text{Let the age of teacher = x years}\\ \text{According to the question,}\\ \text{30 × 9 + x = 31 × 10}\\ \text{270 + x = 310}\\ \text{x = 40 years}\end{array}$


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