Alright, so I'm currently practicing IV drip rates and for the most part I understand them, but I get confused when it's supposed to be run over 20...

Alright, so I'm currently practicing IV drip rates and for the most part I understand them, but I get confused when it's supposed to be run over 20 minutes, 30 minutes instead of hours.  For example: Calculate ml/HR- 30mL of antibiotic in 0.9% NS in 20 minutes by infusion pump...please help!!!