The total volume of a 3-inch note pad is (3)^3 = 27 cu.in. There are 5 total colors, so the yellow portion is 1/5(27) = 5.4 cu.in. If one portion is 5.4, than the others are also 5.4, but instead of writing 5.4 you would write 5.4^3 because there are also three more colours.
