The flow of rate that you are asked for is the number of drops per time unit.
Usually it is counted the number of drops per minute, so I will aim to that.
1) Calculate the total volume.
Assuming additive volumes => final volume = volume of sterile water + volume of the dextrose solution
Final volume = 20 ml + 500 ml = 520 ml.
2) Calculate the number of drops in 520 ml, given that there are 15 drops in 1 ml:
=> 520 ml * 15 drops / ml = 7800 drops.
3) That must be administered over 2 hours, which is 120 minutes.
So, calculate the flow rate as the number of drops divided by the time in minutes:
flow rate = 7800 drops / 120 minutes = 65 drops / minute.
Answer: 65 drops / minute
