The average of $11$ numbers is $10.9$. If the average of the first six is $10.5$ and the average of the last six is $11.4$, find the middle (sixth) number.

David obtained $76, 65, 82, 67$ and $85$ marks (out of $100$) in English,Mathematics,Physics,Chemistry,and Biology. What is his average marks?

The average of three numbers,of which the greatest is $16$,is $12$. If the smallest is half of the greatest,what is the remaining number?

Find the average of all the numbers between $6$ and $34$ which are divisible by $5$.

The cost of $5 \,kg$ of apples is $Rs. \, 450$. The cost of $12$ dozen mangoes is $Rs. \, 4320$,and the cost of $4 \,kg$ of oranges is $Rs. \, 240$. What is the total cost of $8 \,kg$ of apples,$8$ dozen mangoes,and $8 \,kg$ of oranges? (in $Rs.$)

The average salary of the entire staff in an office is $Rs. 120$ per month. The average salary of officers is $Rs. 460$ and that of non-officers is $Rs. 110$. If the number of officers is $15$,then find the number of non-officers in the office?