Mary mixes white paint and blue paint in the ratio 2:5. She makes a total of 21 litres of paint.
How much more blue paint does she need to add to the mixture so that
the ratio of white paint to blue paint becomes 1 : 4?

Add 44 liters blue

Step-by-step explanation: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: