Let the cost of a square inch of paint = p
Let the cost of a cubic inch of concrete = c
The surface area (in square inches) of a 1 inch cube = 6(1)^2 = 6
The volume of 1 inch cube (in cubic inches) = 1^3 = 1
The surface area of a two inch cube = 6(2)^2 = 24
The volume of a 2 inch cube = 2^3 = 8
So.....we have this system of equations
6p + 1c = 1.30 ⇒ c = 1.30 - 6p (1)
24p + 8c = 6.80 (2)
Sub (1) into (2) and we have
24p + 8 ( 1.30 - 6p) = 6.80 simplify
24p + 10.4 - 48p = 6.80
-24p + 10. 4 = 6.80
-24p = -3.6 divide both sides by -24
p = .15
So c = 1.30 - 6(.15) = 1.30 - .90 = .40
So......a 3 inch cube has surface area of 6(3)^2 = 54 in^2
And the volume is (3)^3 = 27 in^3
So.....the cost of a 3 inch cube is
54(.15) +27(.40) = $18.90