In a school with $600$ students,the average age of the boys is $12$ $years$ and that of the girls is $11$ $years$. If the average age of the school is $11$ $years$ and $9$ $months$,then the number of girls in the school is:

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.

The average pocket money of $3$ friends $A, B,$ and $C$ is $Rs. 80$ per month. If $B$ spends double and $C$ spends triple of what $A$ spends during that month,and if the average of their unspent pocket money is $Rs. 60,$ then how much does $A$ spend (in $Rs.$)?

The average age of a family of $6$ members is $22 \text{ years}$. If the age of the youngest member is $7 \text{ years}$,then what was the average age of the family at the birth of the youngest member?

The average of $10$ numbers is $7$. What will be the new average if each of the numbers is multiplied by $8$?

$A, B, C, D,$ and $E$ are five consecutive even numbers. The average of $A$ and $E$ is $46$. What is the largest number?