Triangular Arbitrade Approximation - Will this hold?

Came up with this concept when taking the first CFAI Sample today and tested it out on another practice problem and it seems to hold: 1. Calculate the bid/ask cross rates (or just cross rate if no bid-ask info) 2. Find the difference between this rate and the quoted fx rate 3. Multiple the difference by the nominal amount I know its not exact, but it seems to be within 10% of the answer of correct answer. As long as the answer choices seem spread out, this seems to work.

i don’t know, but just learn the real way, it;s not bad once you get the hang of it