Crossrate calculation formulas - are they correct?

I'm going through all possible calculations to calculate different crossrates, maybe someone can review them and tell me if they are correct?

multiplicate crossrates:

YYYXXX * XXXWWW = YYYWWW


divide crossrates:



YYYXXX / WWWXXX = YYYWWW


XXXYYY / XXXWWW = WWWYYY


1 / XXXYYY = YYYXXX


I guess with these formulas its possible to calculate any desired crossrate, or am I missing a formula?



thanks