One granola bar left out of a pan of 10 bars. If Will & Rachel decide to split the last granola bar, what fraction of the entire pan of granola bars will each friend receive?

So, there's 1/10 of a pan that used to be full off granola, but needs to be split evenly between the two kids. That would end you up with, 1/20. Cross multiply 1/10 with 2/10 and you get 1/20.