Take two portions of 1:2 lemonade and one portion of 1:5 lemonade, mix them evenly, and then measure 400ml of the mixed solution, which is 1:3 lemonade.

Let the first 1:2 lemonade be xml, and the first 1:5 lemonade be 400-xml:

X/3+(400-x)/6=400/4, and the solution is x=800/3ml.