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

In a set of prime and composite numbers,the number of composite numbers is twice the number of prime numbers,and the average of all the numbers in the set is $9$. If the number of prime numbers and composite numbers are exchanged,then the average of the set of numbers increases by $2$. If,during the exchange of the numbers,the average of the prime numbers and composite numbers individually remained constant,then what was the ratio of the average of composite numbers to the average of prime numbers (initially)?

In a class,the average height of all students is $a \text{ cm}$. Among them,the average height of $10$ students is $b \text{ cm}$ and the average height of the remaining students is $c \text{ cm}$. Find the number of students in the class. (Here $a > c$ and $b > c$)

The average of $25$ consecutive odd integers is $55$. The highest of these integers is

$A$ librarian purchased $50$ storybooks for his library. He realized that he could get $14$ more books by spending $Rs. 76$ more,and the average price per book would be reduced by $Rs. 1$. What was the average price (in $Rs.$) of each book he initially bought?

$16$ 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 $B$?