Drop factor: 20 drops per millimeter
15 drops per minute;
20/1 = 15 /x
Solve for x
x = 15/20
x= 0.75 milimeters per minute
So, for 750 ml
750 / 0.75 = 1,000 minutes
since 60 minutes = 1 hour
1,000 / 60 = 16.667 hours
16 hours = 960 minutes
1000-960 = 40
16 hours and 40 minutes