The average age of $24$ boys and their teacher is $15 \,years.$ When the teacher's age is excluded, the average age decreases by $1\, year.$ The age of the teacher is in $years$ ?

A student bought $4$ books for $Rs. \,120$ from one book shop and $6$ books for $Rs.\, 150$ from another. The average price (in rupees), he paid per book was 

In a cricket team of $11$ boys, one player weighing $42 \,kg$ is injured and replaced by another player. If the average weight of the team is increased by $100\, gm$ as a result of this, then what is the weight of the new player? (in $kg$)

The average marks of $120$ students are $35.$ If average of passed students was $39$ and failed students was $15,$ then Find the number of students who have passed?

In an examination the average marks was found to be $60.$ The marks of $80$ candidates had to be changed from $95$ to $70$ each and the average came down to $55$ marks. The total number of candidates attended the examination were