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When I say "...run 29km per day" or "She played music for 2 hours per day during last three month" should I add "on average" at the end of the sentence, or does "per day" already means "on average"?

2 Answers 2

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It depends what you mean.

I ran 29km per day.

means that every day you ran 29km (usually understood to mean 29km or more).

I ran 29km per day on average

means that you can have run sometimes more than 29km, sometimes less, but that the average was 29km per day. Mathematically this would normally be the total of distance run divided by number of days is 29km.

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You can add "on average" to make the meaning more clear. But it isn't required grammatically, and most people would understand from the context.

Saying "I run 29km per day" could mean exactly 29km every day, or it could mean an average of 29km per day. In context, most people don't have a daily running route that is exactly 29 km long, so the meaning of "on average" is understood from the context. But I'd be surprised if someone ran 203km every Sunday and then told me "I run 29 km per day (on average)", even though mathematically it is true.

So "per day" doesn't mean "on average" but in many contexts it will be understood to mean "about and on average, not exactly".

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  • Also, using a round number (30km rather than 29km) is more likely to be taken as an approximation or average.
    – StephenS
    Commented Oct 30, 2020 at 20:33

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