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.
No no, she totally gets what the structure of the problem is, and she can do it for very small numbers. What she can't do is implement an algorithm that will get her the answer -- for example, count up from 3 to 9, while ticking off on her fingers and then see how many fingers she used; or count down from 9 by exactly 3.
