[parseljc]

I think the riddle is stated wrong. There is no need to tell us that the sum is less than 99, even if it is. Giving away too much info!

Plus, I think it goes like this:

Mr. P: "I don't know the numbers."
Mrs. S: "Hmmm, I don't either."
Mr. P: "Now I know the numbers."
Mrs. S: "Oh, now I know them, too!"

Update: I was wrong, the one I heard went like this:
Mr. P: "I don't know the numbers."
Mrs. S: "I already knew you couldn't know them."
Mr. P: "Now I know the numbers."
Mrs. S: "Oh, now I know them, too!"


Which is basically the same as what the one here posted. But telling us the sum is less than 99 is definitely an unnecessary hint.