A batsman has a certain average of runs for $12$ innings. In the $13^{th}$ innings, he scores $96$ runs and thereby increasing his average by $5$ runs. What is his average after the $13^{th}$ innings ?

The average age of $8$ members of a group is $40 \,years.$ The age of $1$ member is $55\, years.$ After his retirement a new member comes in his place of age $39 .$ What is the effect of his retirement on the average age of group ?

The average age of $8$ men is increased by $2\,years$ when two of them whose ages are $21\, years$ and $23\, years$ are replaced by two new men. The average age of the two new men is

The average salary of all workers in a workshop is $Rs.\, 12000 .$ The average salary of $7$ technicians is $Rs.\, 15000$ and the aterage salary of the rest in $Rs.\, 9000$. The total number of workers in the workshop is:

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