Well, trial and error is a good way to get the answer, but more
importantly, kids must understand the reasoning behind the pattern
that allows one to use trial and error since this method relies on
pattern.
Once you get the idea, you'll never need to use trial and error
again. 
Before I learned how to use the pattern formulae (forgot what its
called), I used trial and error when I got stuck, now by deriving
the right formula from a problem, I never needed such a method
again.
