If the standard deviation of $0,1,2,3 \cdots\cdots\cdots9$ is ' $k$ ' then the standard deviation of $10,11,12,13, \cdots\cdots\cdots\cdots19$ will be :

If the mean of $4,7,2,8,6$ and $k$ is 7 . Then the mean deviation from the mean of these observations is

The variance of 25 observations is 8 . If each observation is multiplied by 3 , then the new variance of the resulting observations is