i'd never heard the term before, but a quick net search turned up the answer. the best summary seems to be this one, which came from here.

Quote
Traditionally the "Rule of Three" is the method of solving proportion statements such as:

" A is to B as C is to D "

for one of the four quantities in terms of the other three. The solution is always the product of two of them divided by the third: for instance in the above, we have D = BC/A.
so, if you see A/B = C/D you know that D = (B*C)/A

i can see why the rule isn't often named. we were taught how to manipulate fractions... multiply both sides by B or devide both sides by A or add and subtract or whatever you needed to do. it's just simple algebra. the "rule of 3" seems far too limited an approach.

anyway, hope that answers your question.

Paul

When in doubt, think about penguins. It probably won't help, but at least it'll be fun.