"Hvor mange epler spiser hestene i gjennomsnitt?"
Translation:How many apples do the horses eat on average?
I guess so, though the word order is different from the original without it being necessary.
Agree that having "on average" in the introductory clause is grammatically correct and an acceptable translation.
Does "i gjennomsnitt" also means "in general" as in "In gjennomsnitt kvinner kjøper mer enn menn". Does "kjøper" stand in the right place in my sentence or should it stand after the word "gjennomsnitt"?
No, "i gjennomsnitt" does not also mean "in general". In your sentence "kjøper" should be in front of "kvinner".
Is "the" necessary before "horses" in Norwegian? Is "Hvor mange epler spiser hester i gjennomsnitt?" also a correct sentence in Norwegian?
It is also a correct sentence, but would refer to horses in general rather than a specific group of them.