Question

five years ago mean age of a family was 25 years what is the present mean age of the family

Answer 1

Let the number of family members be 'x'

the average age of the family members 5 years ago is 25 years

so the sum of age of all family members 5 years ago = 25 into x = 25x

the sum of age of these family members now and after 5 years elapsed = 25 x (these added 5 years * no of family members)  = 25x + 5x = 30x

Sum of ages = 30x

no. family members = x

Therefore, current, average age  =  sum of ages/ no. of members = 30x / x = 30

Answer 2

Let the family have X members

Mean=25

Sum of ages=25X

After 5years, all ages are increased by 5

Sum of ages=25X+5(X)=30X

Mean=sum of observations /number of observations,

Mean=30X/X=30