It’s a more straightforward operation than adding. I ultimately end up saying “top times top, bottom times bottom”. That works, but why does it work, when it doesn’t work for adding fractions?
This might help a bit… Reinforces the notion that of means times, i.e. when I buy 4 bags of crisps at 30p each, they cost 4 x 30p. Same for fractions, e.g. 1/5 of 2/3