If the sum of the deviations of 50 observations from 30 is 50 , then the mean of these observations is

Variance of first n natural numbers is ________.

The mean of the numbers $a, b, 8,5,10$ is 6 and the variance is $6.8$ . Then which of the following gives possible values of $a$ and $b$ ?

If for some $x \in \mathbb{R}^{+} \cup\{0\}$, the frequency distribution of the marks obtained by 20 students in a test is

then the mean of the marks is