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.

In a match,the average of runs scored by five players is $49$. If the runs scored by four players are $75, 30, 63,$ and $21$ respectively,then how many runs did the fifth player score?

The average monthly salary of a staff of $9$ persons is $Rs.\, 2450$. One member of the staff whose monthly salary is $Rs.\, 2650$ is transferred. Find out the average salary of the remaining $8$ persons of the staff (in $Rs.$).

Refer to the line graph and answer the given question.
Number of candidates who qualified in a given competitive examination from $6$ states during two given years.
What is the average number of candidates who qualified in the given competitive exam from states $Q, S, T$ and $U$ in $2004$?

The average marks of a group of $20$ students on a test is reduced by $4$ when the topper who scored $90$ marks is replaced by a new student. Find the marks of the new student.

In a family,the average age of a father and a mother is $35 \text{ years}$. The average age of the father,mother,and their only son is $27 \text{ years}$. What is the age of the son (in $\text{years}$)?