# Probability ?

Hi,

Can someone please explain to me the process here? I'm not understanding the rules being used and can't find the answers anywhere

### 1 Answer

- The GnosticLv 74 weeks agoBest Answer
Remember that the standard deviation is the square root of the variance. The explanations are telling you the rules of determining the variances of sums and products of distributions: (1) The variance of k times a distribution is k² times the variance of the (original) distribution, and (2) The variance of the sum of two distributions is the sum of the variances of the individual distributions. Since the variance is the square of the distribution: the s.d. of k times a distribution is k times the s.d. of the distribution, and the s.d. of the sum of two distributions is the square root of the sum of the squares of the s.d. of the individual distributions (kinda like Pythagoras).