If you describe your math test as duck soup, you mean that it was incredibly easy.
There are a lot of figurative ways to say something's simple besides duck soup: easy as pie, piece of cake, and a breeze are just a few. Less easy is figuring out exactly where duck soup came from. It seems to have appeared in the early 1900's in a series of newspaper cartoons, but experts wonder about its origins — and how making duck soup could possibly be considered as easy as falling off a log.