Mary mixes white paint with blue paint in the ratio 2: 3. she makes a total of 20 litres of paint . how much more paint will she need to add to the mixture so that the ratio of the white paint to blue paint will be 1: 7

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:

Add 44 liters blue