18. \( \frac{65 \mathrm{~kg}}{1 \mathrm{~L}}=\frac{x \mathrm{~g}}{1 \mathrm{~cm}^{3}} \)

We are given the equation \[ \frac{65\,\mathrm{kg}}{1\,\mathrm{L}} = \frac{x\,\mathrm{g}}{1\,\mathrm{cm}^3}. \] First, we convert the units to be consistent. 1 liter is equivalent to \(1000\,\mathrm{cm}^3\). Also, \(65\,\mathrm{kg}\) is the same as \(65 \times 1000 = 65000\,\mathrm{g}\). Now, the density in grams per cubic centimeter is \[ \text{Density} = \frac{65000\,\mathrm{g}}{1000\,\mathrm{cm}^3} = 65\,\mathrm{\frac{g}{cm^3}}. \] Therefore, we have \[ \frac{x\,\mathrm{g}}{1\,\mathrm{cm}^3} = 65\,\mathrm{\frac{g}{cm^3}}, \] which implies \[ x = 65. \]