Mary has 20 liters of paint mixed white:blue in the ratio 2:3. So two fifths of that paint is white and three fifths is blue. So that's 8 liters white, 12 liters blue.
We want to add paint to get white:blue = 1:7. Clearly we need to add blue. We have our 8 liters of white, so we need a total of 7(8)=56 liters of blue. We already have 12, so we have to add 56-12=44 liters of blue paint:
