1. Convert Milliliters to Liters

* There are 1000 milliliters (ml) in 1 liter (L).

2. Calculate the Total Milliliters

* 3900 ml + 3100 ml = 7000 ml

3. Divide the Total Milliliters by 1000

* 7000 ml / 1000 ml/L = 7 L

Answer: There are 7 liters in the sum of 3900 milliliters and 3100 milliliters.