The average height of the students of a class of $20$ is $105 \, cm$. If ten more students of average height of $120 \, cm$ join this class,the average height of the class will become (in $cm$)?

Find the average of the following set of scores: $253, 124, 255, 534, 836, 375, 101, 443, 760$.

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 $100$?

In an examination,out of $20$ students in a class,in Mathematics $2$ students scored $100$ marks,$3$ students scored $0$,and the average marks for the rest of the students was $40$. What is the average mark of the whole class? (in marks)

$A$ person covers $9 \, km$ with a speed of $3 \, km/h$,$25 \, km$ with a speed of $5 \, km/h$,and $30 \, km$ with a speed of $10 \, km/h$. Find the average speed of the person in $km/h$.

$A$ class has $20$ boys and $30$ girls. The average age of boys is $11$ years and that of girls is $12$ years. What is the average age of the whole class in years?