Answer :

The 4 litres of red has to be 2 parts of the mixture, so each part will be 2 litres. Therefore 3 parts of cream will be 6 litres and the one part of blue will be 2 litres.
So you have 2 parts red=4 litters, so one part red would be 2 litters making blue two litter and red 3 last equal to six