To start, we need to define what a “solution” is, since you’re measuring that volume. The solution refers to the entire mixture of liquids that Dr. Watson mixes together, so the solution that he creates is a mixture of his detergent, alcohol, and water. The volume of the mixture is determined by the volumes of each of the ingredients and will be the sum of all of the ingredients’ volumes when they are combined. So, the total volume will be:
volume of detergent + volume of alcohol + volume of water
400 mL + 800 mL + 1500 mL
1200 mL + 1500 mL
2700 mL
The total volume of the mixture would be 2700 mL, or 2.7 L (use a conversion between mL and L).
Hope this helps!