Three envelopes contain a total of six bills. One envelope contains two 10$ bills, one contains two 20$ bills and the third contains one 10$ and one 20$ bills. A label on each envelope indicates the sum of the money in one of the other envelopes. Is it possoble to select one envelope and then state the contents of all the envelopes. Which Envelpoe should you slect? Explain your answer.

I think you can figure this one out... figure out what dollar amount would be labeled on each envelope. When you have that information, figure out how many different ways you can come up with the labeled amount using only 2 bills... does that help you?