Today I am going to treat my garden with PlantGrow liquid fertilizer. The instructions on the bottle tell me to dilute 1 part PlantGrow with 9 parts water, then apply evenly. Four hours later the treatment should be repeated, but with a mixture of 1 part PlantGrow and 19 parts water. I have worked out that I need to make up a total of 15 litres of diluted liquid for each application. How much PlantGrow will I use altogether today?