The average salary per head of all the workers in a workshop is $Rs.\,850.$ If the average salary per head of $7$ technicians is $Rs.\, 1000$ and the average salary per head of the rest is $Rs.\, 780,$ find out the total number of workers in the workshop.

A student was asked to find the arithmetic mean of the numbers $3,11,7,9,15,13,8,19,17,21,14$ and $x$. He found the mean to be $12 .$ What should be the number in place of $x ?$

$10$ years ago, the average age of a family of $4$ members was $24$ $years.$ Two children having been born (with age difference of $2\, years$), the present average age of the family is the same. The present age of the youngest child is (in $years$)

The average of nine numbers is $50.$ The average of the first five numbers is $54$ and that of the last three numbers is $52 .$ Then the sixth number is?

The average of $13$ results is $50 .$ If the average of first $7$ results is $52$ and that of last $7$ results is $49,$ find the $7^{th}$ result.