and much more in  

"Advanced Level Statistics" 

available below at amazon.co.uk

Investigation:

With this spreadsheet you can create probability distributions and transform them by adding the random variables X and Y 

to make a new random variable X+Y

Investigate the effect the transforms have on the expected value and the variance.

This is not a QHE question. It's just a bit of fun with probability distributions.

and much more  in  

"Advanced Level Statistics" 

available below at amazon.com