Mary mixes white paint and blue paint in the ratio of 2:3 she makes 20 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 1:7?

The 2:3 ratio has 2+3 = 5 ratio untis. If that total represent 20 litres of paint, then each ratio unit stands for 4 litres of paint. That is, the mixture has

8 litres of white paint
12 litres of blue paint

Mary wants to make the mixture have the ratio 1:7, so the number of litres of paint will be ...

... 1 white : 7 blue = 8 litres of white : 56 litres of blue

That is, the number of litres of blue paint must increase from 12 to 56, an increase of ...

... 44 litres of blue paint