Speaking as a math teacher I ' d suggest instead using volume axconc avolume bxconc b . Dividing volume a by conc b doesn ' t make intuitive senseit has no scientific meaningso it ' s just asking for students to memorize a formula instead of understanding . But setting the quantities equal as the product of concentration and volume is quantity makes sense and can be explained intuitively . So I ' d suggest using that rewrite to transition students from a memorization approach to an understanding approach . And if you had done this , you wouldn ' t have made the error on the board because the insensibility of what you wrote would have been immediately obvious . Basically , don ' t teach formulas . Teach sensemaking .