"How many pesos do you have to pay?"
Translation:¿Cuántos pesos tienes que pagar?
Tener means to have, but tener que + infinitive means have to + infinitive. :)
Tengo trece gatos. - I have three cats.
Tengo que leer el libro. - I have to read the book.
Hope this helps.
I think it's the absence of clues that is the clue. I get dinged when the sentence says "sir" and i use the informal. So if it's not "sir" or "ma'am" it's probably safe to assume it's not formal.