In a class,there are $20$ boys whose average age is decreased by $2$ months,when one boy aged $18$ years is replaced by a new boy. The age of the new boy is:

The average age of a group of $18$ persons is $32$ $years$ and $6$ $months$. If two persons of age $65$ $years$ each and one person of age $50$ $years$ leave the group,the average age of the remaining persons in the group will be (in $years$)

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

$A$ student finds the average of ten $2$-digit numbers. While copying the numbers,by mistake,he writes one number with its digits interchanged. As a result,his answer is $1.8$ less than the correct answer. The difference between the digits of the number in which he made the mistake is:

An aeroplane flies along the four sides of a square field at speeds of $200, 400, 600$ and $800 \, km/hr$. The average speed of the plane in the flight around the field in $km/hr$ is:

In a family,the average age of father and mother is $38 \text{ years}$,whereas the average age of father,mother,and their only daughter is $28 \text{ years}$. Then,the age of the daughter is? (in $\text{years}$)