The average age of $20$ boys in the class is $15.6\, years.$ Five new boys join and the new average becomes $15.56\, years.$ What is the average age of the five new boys? (in $years$)

The average age of $x$ and $y$ is $20$ $years.$ If $z$ were to replace $x,$ the average would be $19$ and if $z$ were to replace $y$, the average would be $21.$ Find the ages of $x, y$ and $z$.

The average salary per head of all workers of an institution is $Rs. \,60\,.$ The average salary per head of $12$ officers is $Rs. \,400 .$ The average salary per head of the rest is $Rs.$ $56 .$ Then, the total number of workers in the institution is?

$10$ years ago, the average age of a family of $4$ members was $24$ $years.$ Two children having been born (with age difference of $2\, years$), the present average age of the family is the same. The present age of the youngest child is (in $years$)

Average temperature of first $4 \,days$ of a week is $38.6^{\circ} C$ and that of the last $4\, days$ is $40.3^{\circ} C$. If the average temperature of the week be $39.1^{\circ} C$, the temperature on $4^{th}$ day is. (in $^{\circ} C$)

The average of runs of a cricket player of $10$ innings was $32 .$ How many runs must he score in his next innings so as to increase his average of runs by $4 ?$