Let the parts of red pigment required to mix with 1800 mL of base be x.
The given information in the form of a table is as follows.
|Parts of red pigment||1||x|
|Parts of base (in mL)||75||1800|
The parts of red pigment and the parts of base are in direct proportion.
Therefore, we obtain
Thus, 24 parts of red pigments should be mixed with 1800 mL of base.