Learn Farsi: ما یک ربع دیگر می‌رسیم. - We will arrive in a quarter of an hour.

Rob' means the fraction "one quarter" (1/4) in general. When used in the context of time, it is universally understood to mean a quarter of an hour (15 minutes). Because it's so common, you don't even need to include the word for "hour" (sa'at) here.

Why do we use دیگر (digar) here to mean "in"?

Digar literally means "other" or "another". So, yek rob' digar translates literally to "another quarter". While in English you say you will arrive "in" a certain amount of time, Farsi expresses this as arriving after "another" chunk of time passes.

Could I say fifteen minutes instead of یک ربع (yek rob')?

Yes, absolutely! You can say پانزده دقیقه (panzdah daqiqeh), which means "fifteen minutes". Saying panzdah daqiqeh digar is just as natural and correct as saying yek rob' digar.

Why is the verb می‌رسیم (miresim) in the present tense if the English translation uses "will"?

می‌رسیم is the present tense of "to arrive", but Farsi very often uses the present tense to talk about the near, definite future. This is exactly like saying "We are arriving in 15 minutes" or "We arrive in 15 minutes" in English.