The average marks of a student in four subjects is $75$. If the student obtained $80$ marks in the $5^{th}$ subject,then the new average is?

In a cricket team of $11$ boys,one player weighing $42 \,kg$ is injured and replaced by another player. If the average weight of the team is increased by $100 \,g$ as a result of this,then what is the weight of the new player (in $.1$)? (in $kg$)

$A$ student was asked to find the arithmetic mean of the numbers $3, 11, 7, 9, 15, 13, 8, 19, 17, 21, 14$ and $x$. He found the mean to be $12$. What should be the number in place of $x$?

The average of certain first consecutive natural numbers is $20.5$. Find their sum.

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 of five numbers is $27$. If one number is excluded,the average becomes $25$. The excluded number is