An administration set delivers 60 drops to the ml. How many drops per minute are needed to obtain 20 units of Heparin per minute if the IV admixture contains 15,000 units per 250ml of Normal Saline

An administration set delivers 60 drops to the ml. How many drops per minute are needed to obtain 20 units of Heparin per minute if the IV admixture contains 15,000 units per 250ml of Normal Saline?
a- 20***
b- 40
c- 60
d- 80
e- 120.