If the mean of 4 observations is 20, when a constant 'C' is added to each observation, the mean becomes 22. The value of C is: 

The average of the squares of first ten natural numbers is 

Find mean of first five prime number.

The mean of 14 observations is 11. One more observation is included and the new mean becomes 12. The  observation is

The average age of 30 boys in a class is 15 years. One boy, aged 20 years, left the class, but two new boys came in his place whose age differs by 5 years. If the average age of all the boys now in the class becomes 15 years, the age of the younger newcomer is: