Amitabh's average expenditure from $January$ to $June$ is $Rs. 4200$. He spends $Rs. 1200$ in $January$ and $Rs. 1500$ in $July$. What is the average expenditure for the months of $February$ to $July$ (in $Rs.$)?

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

The arithmetic mean of the scores of a group of students in a test was $52$. The brightest $20\%$ of them secured a mean score of $80$ and the dullest $25\%$ a mean score of $31$. The mean score of the remaining $55\%$ is

The average of $30$ numbers is $15$. The average of the first $18$ numbers is $10$ and that of the next $11$ numbers is $20$. The last number is:

The average salary of the entire staff in an office is $Rs. 500$ per day. The average salary of officers is $Rs. 750$ and that of non-officers is $Rs. 250$. If the number of officers is $15$,then find the number of non-officers in the office?

The average marks of a group of $20$ students on a test is reduced by $4$ when the topper who scored $90$ marks is replaced by a new student. Find the marks of the new student.