There's currently 20 liters of paint that's 2 parts white and 3 parts blue.
This tells us that there is 8 liters of white and 12 liters of blue.
We need paint that's 1 part white and 7 parts blue.
Which means we need 8 liters of white and 8(7) liters of blue.
This amounts to a total of 56 liters of blue.
We already have 12, so we need to add an additional 44 liters of blue paint.
