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
$\begin{array}{l}\text{Let the age of teacher = x years}\\
\text{According to the question,}\\
\text{30 × 9 + x = 31 × 10}\\
\text{270 + x = 310}\\
\text{x = 40 years}\end{array}$
