Conventions cannot be discovered. Just tell the learners that the convention exists.
The key thing here is that you look at any piece of algebra you use and investigate yourself what conventions are involved. Conventions that are known to you are probably not known by your learners. For example, suppose you have a formula like where n is nett weight and g is gross weight (formula for yield). In number means the number three-eighths, it does not mean 3 ÷ 8 though is the result of the calculation 3 ÷ 8 = ? So your learners will not automatically know that one meaning of is 40g ÷ 100.
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