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 $\text{years}$)?

The average value of the numbers $15, 21, 32, 35, 46, x, 59, 65, 72$ should be greater than or equal to $43$ but less than or equal to $44$. Then the value of $x$ should be

Of the three numbers whose average is $60$,the first is one-fourth of the sum of the others. The first number is

The average age of $20$ boys in the class is $15.6$ years. Five new boys join and the new average becomes $15.56$ years. What is the average age of the five new boys (in years)?

Find the average of all prime numbers between $60$ and $90$.

The average salary per head of all workers of an institution is $Rs. 60$. The average salary per head of $12$ officers is $Rs. 400$. The average salary per head of the rest is $Rs. 56$. Then,the total number of workers in the institution is?