Question

In a group of $20$ students the average age of the students is $10$ years. A boy leaves the group and the average is decreased by $1$ year. Find the age of that boy.

Answer 1

$\Rightarrow$ The average age of $20$ students is $10years$

$\therefore$ Sum of age of $20$ students $=20\times 10=200years$

$\Rightarrow$ A boy leaves the group and the average is decreased by $1$ year, means average age of $19$ students is $9$.

$\therefore$ Sum of age of $19$ students $=19\times 9=171years$

$\Rightarrow$ The age of that boy who leaves the group $=200-171=29years$