It's written a little oddly, but makes sense. I can show you how to do it in Excel, but since you want to understand it first, the best way to start...
First, please let me know if this belongs in another forum category, but it seemed like probability/statistics was the closest match to my problem. ...
I've seen this problem before, given to students at this level, probably intended to be solved by orderly trial and error, with a little intelligence...