4 years ago, the average age of a family of 7 members was 35yr. The present average age is also 35 year after including a child into a family. Find the age of the child.
12 year
14 year
9 year
7 year
Solution
Total age of 7 members (4 years ago) $=~35\times 7~=~245$
Total age of 7 members (at present) $=~245~+~7\times 4~=~273$
Let the present age of the child = x years
$x~+~273~=~35\times 8$
$x~=~280~-273$
$x~=~7$ years