The mean deviation about the mean for the date $4,7,8,9,10,12,13,17$ is

Let $a, b, c, d$ and $e$ be the observations with mean $m$ and standard deviation $S$. The standard deviation of the observations $a+k$, $b+k, c+k, d+k$ and $e+k$ is

The mean of 100 observations is 50 and their standard deviation is 5. Then, the sum of squares of all observations is

If the standard deviation of the numbers $$-1, 0,1, k$$ is $$\sqrt{5}$$ where $$k>0$$, then $$k$$ is equal to