Discussion
The average age of 20 number students in a class is 11 years. If the age of teacher is added, the average age is increased by 1 year. What is the age of the teacher ?
Answer: A
The teacher is 32 years old.
Work with totals. The 20 students' ages add to \(20 \times 11 = 220\). With the teacher there are 21 people averaging 12 years.
\[ \begin{aligned} \text{new total} &= 21 \times 12 = 252 \\ \text{teacher} &= 252 - 220 = 32 \end{aligned} \]
Shortcut: the teacher brings his own share (12) plus 1 year for each of the 20 students, \(12 + 20 = 32\). 36 and 42 fit no step, so 'None of the above' is wrong too.
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