Why is the preposition 'a' is used with this comparison?

Question

I noticed these two sentences used the preposition 'a' instead of 'que' in these sentences. Is there a reason why? 
Las ventas fueron superiores a los costes.
The sales were higher than the expenses.

Los costes fueron inferiores a las ventas.
The expenses were lower than the sales.

Answer

Hola Amy
With adjectives "inferior" and "superior" used in a comparative sentence we use preposition "a", not "que". 
However, with adjectives "menor" and "mayor", despite meaning pretty much the same as the previous ones,  we need to use "que", not "a".
I hope this helps.
Inma

Answer

Thanks! I just wanted to know the specific times it is used, as it wasn't mentioned in the lesson :)