The average weight of $36$ students is $50 \,kg$. If was found later that the weight of one of the student was misread as $73 \,kg$. whereas his actual weight was $37 \,kg$. Find the correct average. (in $kg$)

A man travels by a car to his office at $60 \,km / h$ and returns home along the same route at $20 \,km / h .$ Find the average speed of his whole journey. (in $km/h$)

The average age of a family of $6 \,members\, 4 \,year$ ago was $25 \,years.$ Mean while a child was born in this family and still the average age of the whole family is same today. The present age of the child is in $years$ $?$

What is the average of the squares of the first $19$ natural numbers ?

The arithmetic mean of $100$ observations is $24.6$ is added to each of the observations and, then each of them is multiplied by $2.5 .$ Find the new arithmetic mean.

An aeroplane travels $2500 \,km , 1200 \,km$ and $500\,km$ at $500\, km / h \,, 400 \,km $ and $250\,km / h $ respectively. The average speed is:(in $km/h$)