A class has $20$ boys and $30$ girls. The average age of boys is $11\, years $ and that of girls is $12\,years.$ What is the average age of the whole class in $year$

Visitors to a show were charged $Rs.\,15.00$ each on the first day, $Rs.\, 7.50$ on the second and $Rs.\, 2.50$ on the third day. Visitors total attendance for three days were in the ratio $2: 5: 13$ . Find out the average charge per visitor for the entre show. (in $Rs.$)

Average rainfall on Monday, Tuesday, Wednesday and Thursday is $420.5\, cm$ and average on Tuesday, Wednesday, Thursday and Friday is $440.5\, cm .$ If the ratio of rainfall for Monday and Friday is $20: 21,$ find the rainfall in $cm$ on Monday and Friday.

The average of $7$ numbers is $5 .$ If the average of first six of these numbers is $4,$ the seventh number is

The average salary of $20$ workers in an office is $Rs.\, 1900$ per $month.$ If the manager's salary is added, the average becomes $Rs.\,2000$ per $month.$ The manager's annual salary (in $Rs.$) is

$10$ $years$ ago the average age of all the $25$ $teachers$ of the Girls college was $45\, years.$ $4 \,years$ ago, the principal has retired from her post at the age of $60 \,year.$ So after one year a new principal whose age was $54 \,years$ recruited from outside. The present average age of all the teachers is, if principal is also considered as a teacher