Out of 30 teachers of a school, a teacher of age 60 years retired. In his place another teacher of age 30 years was appointed. As a result, the mean age of the teachers will

initial number of teachers = 30

Let the initial average age be z years

Average = $$\frac{sumofages}{numberofperson}$$

So total initial age = 30z

Now it is given that a teacher of 60 years is replaced by 30 years .So

Total final age of teachers = 30z -60+30 = 30z-30

Number od teachers = 30

New average = $$\frac{30z-30}{30}$$ = z-1

And hence we can say that due to given replacement the average age will decrease by 1