The mean age of 25 students in a class is 12. If the teacher’s age is included, the mean age increases by 1. The teacher’s age (in years) is.

Given the mean of  25 students in a class is 12

Sum of observations $=12\times 25=300$

Given that the teacher’s age is included

Let the number included be $x$ .

Hence the mean increases by 1

Hence the sum observation  is $300+x$

New observation is 26

$\frac{300+x}{26}=13\Rightarrow x=338-300=38$