The average age of $\displaystyle 15$ numbers in a class is $\displaystyle 9$ years. Out of them, the average age of $\displaystyle 5$ students is $\displaystyle 13$ years and that of $\displaystyle 8$ students is $\displaystyle 5$ years. What is the average of remaining $\displaystyle 2$ students?

If the range of $\displaystyle 28, 22, 40, 20, 15, 50$ is

If the S.D. of the $\displaystyle 1^{st}$ $\displaystyle n$ natural numbers is $\displaystyle \sqrt{30}$ then the value of $\displaystyle n$ is

If the variance of $\displaystyle 5, 7, 9$ and $\displaystyle 11$ is $\displaystyle 4$, then the coefficient of variation is:

A helicopter flies from A to B at the rate of 500 km/hr. and comes back at the rate 700 km/hr. The average speed of the helicopter is

If Arithmetic Mean (A.M.) and Geometric Mean (G.M.) of two numbers are 6.50 and 6 respectively, then the two numbers are

Which of the following is not a method of dispersion?