Word Usage: Five minutes a day (NOT five minutes per day). What is the difference between "a" and "per"?

Below is an excerpt from a book:

"Revise for short periods but do it often. Five minutes a day (NOT five minutes per day)"

Why can't we use "per"? What is the difference between "a" and "per"?


Without more context I have no idea why the author of the book makes it sound like "times a day" is different in meaning from "times per day". In my English (SE Pennsylvania) they mean the same. In normal, everyday usage "a day" is more common. Maybe the author, simply means to say that an English language learner should learn to use "a day", because it is more normal, common usage than "per day".

I'd have to think about it a litte more, but I think I use "per" with time expressions when measuring frequencies: rotations per minute, miles per hour, but use "a" when just talking about frequency of events, but not measuring anything: He visits his grandmother twice a year. We eat beans three times a week. I might do some mathematical calculations with the rpm and mph frequency measurements, but not with the visiting and eating frequency references.



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