State how many mole ratios can be written for a reaction involving three substances.

It's one ratio per substance so say if you had N2 + 3H2 > 2NH3 (chemical equation for the synthesis of ammonia) just look at the number coefficients(for 3H2 the coefficient would be 3  and for 2NH3 the coefficient would be 2 and so on)So it  would take 1mole of nitrogen to react with 3 moles of hydrogen In order to produce 2 moles of ammonia  so there would be 6 ratios you could write comparing two substances at a time In a reaction involving 3 substances ^^; hope this helps somewhat ^^;