A fanatic for numbers has set the combination to his home safe, leaving his wife in a desperate situation when she has an emergency and needs to get in. Fortunately, from the list of old combinations, Griswold is able to determine the current combination.
OK, the situation here is incredibly unlikely. Asimov sets up a really elaborate back-story that gets more and more complicated as he tries to set up something that will give him his puzzle. The puzzle itself is quite nice, however, and if I'd been paying attention, I'd have been able to figure it out myself.