The teacher tells the class to cross multiply and multiply by the reciprocal. It is easier to say to multiply both sides of the equation by the same number. This is valid by the property of equality: a = a. We can also multiply an expression by 1, where we can write 1 as a / a. This approach involves less steps and is clearer, as it follows from the axioms.

 

M said that the students learn in lower grades about cross multiplying. However, once we properly understand the properties of numbers, this concept is redundant and error prone. We may mention to the class that we will do things differently and better, and not use cross multiplication.