I was dealing with a more general variant of the version that Vos Savant gave. Which is that you were able to look at the contents of the envelope that you got. She claimed you didn't gain useful information. Calling it "information" is problematic, but it certainly is useful, because the stated strategy then works!

This is one reason that probability theorists have learned to be more careful in stating the double money version to rule out the solution that I gave. But my version has been in the literature since, I believe the 1950s. Under, as I first encountered it, the same exact name. (I encountered it from Dr Laurie Snell at Dartmouth College in the mid 1990s.)