Construct a Dutch book against someone who violates the Axiom of Additivity.

That is, suppose a Mr. Fuzzy declares about the weather tomorrow that P(Sunny) =

0:5;P(Inclement)=0:5; and P(Sunny or inclement)=0:5. Mr. Fuzzy and you agree about what will count as sunny and as inclement weather and you both agree that they are incompatible states. How can you construct a Dutch book against Fuzzy, using only fair bets?