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

If the difference between the average of $x, y$ and $y$, $z$ is $12,$ then the difference between $x$ and $z$ is

The average salary of $15$ persons is $Rs.\, 5,500.$ If the salary of one person is added, the average increase to $Rs. \,5,700 .$ What is the salary of this one person in $Rs. \, ?$

The mean of marks secured by $50$ students in division $A$ of class $X$ is $61,\,25$ students of division $B$ is $57$ and that of $50$ students of division $C$ is $55 .$ What will be the mean of marks of the students of three divisions of Class $X$ ?

There were $35$ students in a hostel. If the number of students is increased by $7,$ the expenditure on food increase by $Rs. \,42$ per $day$ while the average expenditure of students is reduced by $Rs.\, 1.$ What was the initial expenditure on food per day? (in $Rs.$)

The average of $17$ numbers is $10.9 $ If the average of first nine numbers is $10.5$ and that of the last $9$ numbers is $11.4,$ the middle number is