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

W:B = 2:5 Total of 21 litres of paint. We want a ratio of W:B = 1:4 = 2:8
Therefore we must add 3 parts of blue paint
2 + 5 = 7 parts total7 parts = 21L1 part = 3L3 parts = 9L
Therefore must add 9L of blue paint