A person travelled from one place to another at an average speed of $40 \,km / h$ and back to the original place at an average speed of $50\, km / h$. What is his average speed in $km / h$ during the entire trip?
$45$
$20 \sqrt{5}$
$400 / 9$
$50$
In a school with $600$ students, the average age of the boys is $12$ $years$ and that of the girls is $11\, years.$ If the average age of the school is $11 \,years$ and $9 \,months,$ then the number of girls in the school is
The average monthly salary of a staff of $9$ persons is $Rs.\,2450.$ One member of the staff whose monthly salary is $Rs.\,2650$ is transferred. Find out the average salary of the remaining $8$ persons of the staff.(in $Rs.$)
The average age of $8$ men is increased by $2\,years$ when two of them whose ages are $21\, years$ and $23\, years$ are replaced by two new men. The average age of the two new men is
The average age of a man and his two twin born on the same day is $30 \,year.$ The ratio of the ages of father and one son is $5: 2,$ what is the father's age in $year?$
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 $cm$)