Given, 45 ml base is added to 1 part of blue pigment.
So, 1 ml of base is added to 1/45 parts of blue pigment.
900 ml of base is added to 1/45 x 900 = 20 parts of blue pigment.
900 x 2 ml of base is added to 1/45 x 900 x 2 = 40 parts of blue pigment.
So, when the base is doubled, the amount of pigment required is also doubled.