 When a law holds for all numerical values of its variables, it also holds for all expression values of those variables. Setting \(x = M, y = 4\pi r^3\), and \(z = 3\) in the last law of the preceding paragraph yields \(M/(4\pi r^3 /3) = 3M/(4\pi r^3)\). The left hand side being our density formula from the preceding section, it follows from this instance of the above law that its right hand side is an equivalent formula for density in the sense that it gives the same answers as the left hand side. This new density formula replaces one of the two divisions by a multiplication.