The average age of a group of $9$ students is $20\, \text{years}$. When $6$ more students joined the group,the average age falls by $2\, \text{years}$. The average age of the new students is:

The average weight of $15$ oarsmen in a boat is increased by $1.6\, kg$ when one of the crew,who weighs $42\, kg$,is replaced by a new man. Find the weight of the new man (in $kg$).

The average salary of the entire staff in an office is $Rs. 500$ per day. The average salary of officers is $Rs. 750$ and that of non-officers is $Rs. 250$. If the number of officers is $15$,then find the number of non-officers in the office?

If the average of $m$ numbers is $n^{2}$ and that of $n$ numbers is $m^{2}$,then the average of $(m + n)$ numbers is

The average of three numbers is $154$. The first number is twice the second and the second number is twice the third. The first number is

It rained as much on Wednesday as on all the other days of the week combined. If the average rainfall for the whole week was $3 \, cm$,then how much did it rain on Wednesday? (in $cm$)