Question

the average age of 30 stydents is 12 if teacher is included the average become 13 find teachers age

Answer 1

Assume x be the Age of the student.
Average age of 40 students = 12 years
Total age is :
40x = 12*40
40x = 480 -> Equation 1 
Let teachers age be y
Average age of 41 people (40 studenets + 1 teacher) = 13 years
Hence total age 
40x + y = 533 -> Equation 2 
Subtracting equation 1 from equation 2, we get
y = 53 
Age of teacher = 53 years 

Answer 2

a+b+......30/30=12
a+b+....30=360
Now when teacher is included
a+b+c+......31/31=13
a+b+c+.....31=403

So age of teacher will be 403-360=43 yrs

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