The average age of a class of 40 students and a teacher reduces by 0.5 years, if we exclude the teacher. If the initial average age is 18 years, then what is the age of the teacher?

If the average weight of 6 students is 50 kg; that of 2 students is 51 kg; and that of other 2 students is 55 kg; then the average weight of all students is

In a class, there are certain numbers of students. Their average age is 12 years. If two students leave the class, their ages are 23 years and 25 years respectively. Then the average age of the class becomes 11 years. Find the number of students in the class.

The sum of the present ages of a father and son is 52 years. Four years hence, the son's age will be $${1\over 4}$$that of the father. What will be the ratio of the ages of the son and father, 10 years from now?

Six years ago, the ratio of ages of A to B was 7 : 5. After 4 years from now, the ratio of their ages will be 11 : 9. What is A’s age at present?

In the first 10 overs of a cricket game, the run rate was only 7.2. What should be the average run rate in the remaining 40 overs to reach the target of 272 runs? 

The average of the squares of first ten natural numbers is 

Find mean of first five prime number.