EN or A? 2

A with times, prices, and rates — A is used with clock times (a las cuatro, al mediodía), with prices (a tres euros el kilo), and rates (100 km a la hora). These uses are fixed expressions — don't try to use en here.

En with specific means of expression — En appears in several fixed expressions where English uses other prepositions: en voz alta/baja (aloud/quietly), en serio (seriously), en broma (jokingly), en resumen (in summary), en total (in total). Learn these as fixed phrases.

A vs en with some verbs — fixed collocations — Some verbs take a fixed preposition regardless of movement: pensar en (to think about), confiar en (to trust), insistir en (to insist on), entrar en (to enter — even though it implies motion). These must be learned as vocabulary items, not derived by logic.