1. **State the problem:** We need to find how long it will take to fill an 800L tank with propane if the truck pumps 20L per minute. 2. **Formula used:** Time = \frac{Total Volume}{Rate} 3. **Apply the values:** $$\text{Time} = \frac{800L}{20L/min}$$ 4. **Simplify the fraction:** $$\text{Time} = \frac{\cancel{800} \times 1}{\cancel{20} \times 1} = 40 \text{ minutes}$$ 5. **Answer:** It will take 40 minutes to fill the 800L tank at the rate of 20L per minute.