In Spain, we do say "menos veinticinco", "menos veinte", "menos cuarto" and so on. No one would say that's "doing math" though, you just end up learning by heart that seeing 40 is "menos veinte", just like you see 30 and don't have to go "oh, so an hour is 60 minutes, so 30 is half that, so it must be half-past ten". It's not uncommon to simply say the number, though, so you can use that, but, at least here, you will hear the "menos whatever" structure very often. More on reddit.com

Cause informal seems way more practical than having to subtract in the formal variation Normally, that's the most common way to say it and it doesn't have to do with formality. The substraction is rarely (if ever) used by Mexicans. We only express how long until the next hour comes but this usually happens after the 35 minute mark every five minutes for easier maths. We also use "y" only at the 15 and 30 minute marks to express y cuarto and y media; other than that I don't think it's common but we do use con although it still sounds a little formal: 12:00: Son las doce; son las doce en punto; es mediodía [noon]; es medianoche [midnight] 12:01: Son las doce uno 12:09: Son las doce nueve 12:15: Son las doce y cuarto; son las doce quince 12:25: Son las doce veinticinco 12:30: Son las doce y media; son las doce treinta 12:35: Son las doce treinta y cinco; son veinticinco para la una; faltan veinticinco para la una 12:42: Son las doce cuarenta y dos 12:45: Son cuarto para la una; son las doce cuarenta y cinco; son cuarto para la una; falta un cuarto para la una; faltan quince para la una 12:50: Son las doce cincuenta; son diez para la una; faltan diez para la una More on reddit.com