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

The average of $20$ numbers is $30$ and that of other $30$ numbers is $50$. What is the average of all the numbers?

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 $X$?

Of the three numbers whose average is $60$,the first is one-fourth of the sum of the others. The first number is

The arithmetic mean of $100$ observations is $24$. If $6$ is added to each of the observations and then each of them is multiplied by $2.5$,find the new arithmetic mean.

The average of $8$ numbers is $20$. The average of the first two numbers is $15 \frac{1}{2}$ and that of the next three is $21 \frac{1}{3}$. If the sixth number is less than the seventh and eighth numbers by $4$ and $7$ respectively,then the eighth number is: