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 marks of a group of $20$ students on a test is reduced by $4$ when the topper who scored $90$ marks is replaced of a new students. Find the marks of the new student.

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?

The average age of a group of $15$ students is $10 \,years.$ When $5$ more students joined the group, the average got increased by $1\, year.$ The age of the new student is (in $years$)

The average of runs scored by a cricketer in his $99$ innings is $99.$ How many runs will he have to score in his $100^{\text {th }}$ innings so that his average of runs in $100$ innings may be $100 ?$

In a coconut grove, $(x+2)$ trees yield $60$ $nuts$ per $year,$ $x$ trees yield $120\, nuts$ per $ year,$ and $(x-2)$ trees yield $180 \,nuts$ per $year$. If the average yield per $year$ per tree be $100,$ find the value of $x .$