Let the ratio of the parts of red pigment and the parts of base be ab.
So, according to direct proportion,
a1b1=a2b2=a3b3=a4b4=a5b5
Let a1b1=18, a2 = 4 and b2 be its respective parts of base and so on.
∴ a2b2=a3b3=a4b4=a5b5=18
a2b2=4b2=18
⇒b2=32
a3b3=7b3=18
⇒b3=56
a4b4=12b4=18
⇒b4=96
a5b5=20b5=18
⇒b5=160