The average of the first nine prime numbers is

The mean of $11$ numbers is $35$. If the mean of the first $6$ numbers is $32$ and that of the last $6$ numbers is $37$, then find the sixth number.

The average age of four members of a family is $32$ years. If the age of a guest is included,then the average age increases by $12.5 \%$. What is the age (in years) of the guest?

The average age of the boys in a class is $16 \text{ years}$ and that of the girls is $15 \text{ years}$. The average age for the whole class is:

The sum of the ages of father and son at present is $33 \, \text{years}$. Two years ago, the product of their ages was $28 \, \text{years}$. What is the age of the father and the son (in $\text{years}$)?

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?