The average age of three boys is $15$ $year.$ If the ratio of their ages is $3:5: 7,$ what is the age of the oldest boy in $year$?

$16$ children are to be divided into two groups $A$ and $B$ of $10$ and $6$ children, respectively. The average marks obtained by the children of group $A$ is $75$ and that of all the children is $76 .$ Then, the average marks of the children of group $B$ is ?

The average of the price per $kg$ of rice at $10$ different places was $Rs.\, 4.85.$ After a week, the price per $kg.$ was increased by $20$ paise at $3$ places and decreased by $10$ paise at one place. The new average of price per $kg$ is in $Rs.$

The average expenditure of a man for the first five months is $Rs.\, 3600$ and for next seven months it is $Rs.\, 3900 .$ If he saves $Rs.\, 8700$ during the $year,$ his average income per $month$ is (in $Rs.$)

Out of seven given numbers, the average of the first four numbers is $4$ and that of the last four numbers is also $4 .$ If the average of all the seven numbers is $3,$ then fourth number is?

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