To calculate the drop rate, we need to determine the number of drops per minute.

First, we need to convert 8 hours to minutes: 8 hours * 60 minutes/hour = 480 minutes

Next, we need to calculate the total number of drops: 1000 mls * 21 drops/ml = 21,000 drops

Finally, we divide the total number of drops by the total number of minutes to find the drop rate: 21,000 drops / 480 minutes = 43.75 drops/minute

Therefore, the drop rate is approximately 43.75 drops/minute.