You might tell them to create some sort of data structure that, using a customer number as a key, would hold their name, address, and phone number - or something along those lines. Let them sweat that for a while, then magically show them how utterly simple refs are (well, OK, maybe not so simple, but feasible). It seems like it's usually a good idea to let students sweat a problem for a while, to give them an idea of its difficulty, before presenting them with a decent solution.