"There are seven days in a week."

Translation: एक हफ़्ते में सात दिन होते हैं।

[Lalremruat13]

Why is it hafte and not hafta?

[GraysonElliott]

The में makes it oblique.

[inbetweener]

Why hote hai and not just hai?

[manimaran_p]

Sentences which convey an univeral truth will have to end like that

eg. एक साल में बारह महीने होते है

[MahaDidi]

Isn't just हफ़्ते में सात दिन होते हैं। ok as well?

[emrys29]

Correct.

[wurdahmekanik]

Could it also be arranged as "सात दिन एक हफ़्ते में होते हैं" ?

[banamakaja]

Can ,,ek saptah mein saat din hote hain,, also be used?

[vinay92]

Yes. सप्ताह and हफ़्ता are synonyms. Just that one comes from Sanskrit and the other from Persian.
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