If an intravenous infusion runs at 4 mL per hour and the total volume is 100 mL, how long will the infusion last?

• A. 20 hours
• B. 25 hours
• C. 50 hours
• D. 75 hours

Answer: (B)

When an IV infusion runs at a constant rate, the time to deliver a given volume is calculated by dividing the total volume by the infusion rate. Here, the total volume is 100 mL and the rate is 4 mL per hour. Time = 100 mL ÷ 4 mL/hour = 25 hours. You can confirm by multiplying back: 4 mL/hour × 25 hours = 100 mL.

The other numbers don’t fit because they would require different rates to deliver 100 mL in those times (for example, 20 hours would need 5 mL/hour, 50 hours would need 2 mL/hour, and 75 hours would need about 1.33 mL/hour).