In a class of $100$ students, the mean marks obtained in a subject is $30$ and in another class of $50$ students the mean marks obtained in the same subject is $60 .$ The mean marks obtained by the students of two classes taken together is

Ten years ago the average age of $P$ and $Q$ was $20\, years.$ Average age of $P, Q$ and $R$ is $30\, years$ now. After $10 \,years,$ the age of $R$ will be (in $years$)

The average of $41$ consecutive odd numbers is $49 .$ What is the largest number.

The average age of four members of a family is $32\, years.$ If the age of a guest is included, then the average age increased by $12.5 \% .$ What is the age (in $years$) of the guest ?

The average salary of the entire staff in an office is $Rs.\, 500$ per day. The average salary of officers is $Rs.\, 750$ and that of non-officers is $Rs. \,$ $250 .$ If the number of officers is $15,$ then find the number of nonofficers in the office?

In a set of prime and composite numbers, the composite numbers are twice the number of prime number and the average of all the numbers of the set is $9 .$ If the number of prime numbers and composite numbers are exchanged then the average of the set of numbers is increased by $2 .$ If during the exchange of the numbers the average of the prime numbers and composite numbers individually remained constant, then the ratio of the average of composite numbers to the average of prime numbers (initially) was