The average of first three numbers is double of the fourth number. If the average of all the four numbers is $12,$ find the $4^{th}$ number.

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 $kg$)

The average of runs scored by a cricketer in his $99$ innings is $99$. How many runs will he have to score in his $100^{\text{th}}$ innings so that his average of runs in $100$ innings may be $100$?

The average age of the boys in a class is $16 \text{ years}$ and that of the girls is $15 \text{ years}$. The average age for the whole class is:

The average of five consecutive positive integers is $n$. If the next two integers are also included,the average of all these integers will

The average weight of $10$ students is increased by $0.5 \, kg$ when one of the students weighing $50 \, kg$ is replaced by a new student. Find the weight of the new student (in $kg$).