The average of $25$ results is $20$. The average of the first $12$ results is $15$ and that of the last $12$ results is $18$. Then,the $13^{th}$ result 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:

The mean of the marks obtained by $100$ students is $60$. If the marks obtained by one of the students was incorrectly calculated as $75$ whereas the actual marks obtained by him were $65$,what is the correct mean of the marks obtained by the students?

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 of $8$ numbers is $20$. The average of the first two numbers is $15 \frac{1}{2}$ and that of the next three is $21 \frac{1}{3}$. If the sixth number is less than the seventh and eighth numbers by $4$ and $7$ respectively,then the eighth number is:

The average age of the boys in a class is $16 \text{ years}$ and that of the girls is $15 \text{ years}$. The average age for the whole class is: