The average age of 10 students and their class teacher is 17 years. If the age of the class teacher is excluded, the average age of the 10 students is reduced by 2 years. What is the age of the class teacher?

37 years

Sum of ages of class including class teacher = 11 x 17 = 187

Sum of ages of class excluding class teacher = 10 x 15 =150

Age of class teacher = 37