One additional point to make users aware of is that once you get one of the error messages such as #DIV/0! it propagates to all the other formulae that use it. To help track it back click in the Formula Bar. All the cells used in the formula are then highlighted and you can see if any of them have the error. If they do you have to look for the problem there.
In general this technique of clicking in the Formula Bar and checking which cells are highlighted (used) is often useful for checking a formula. It doesn't help if the cell is on a different sheet or a long way away. At that point you have to teach your user about Precedents but that is a little advanced for this article.
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