She's getting implied subtraction problems: Basically A + ___ = B. It seems like a weird huge leap to throw at her.
Rather than teach her a strategy, can you reframe the problem for her? Explain that the computer has a secret number, but is giving her a clue: A plus the secret number equals B. Can she find out what the computer's secret number is? Put like that DS was doing this kind of thing ridiculously young - may be just his weirdness, but I suspect it's actually that it's not hard if you look at it that way!
