\(\begin{array} {ll} {\dfrac{x}{4} = \dfrac{20}{16}} & {\text{ means that } 16 \cdot x = 4 \cdot 20} \\ {\dfrac{4}{x} = \dfrac{16}{20}} & {\text{ means that } 4 \cdot 20 = 16 \cdot x} \\ {\dfrac{5}{4}...\(\begin{array} {ll} {\dfrac{x}{4} = \dfrac{20}{16}} & {\text{ means that } 16 \cdot x = 4 \cdot 20} \\ {\dfrac{4}{x} = \dfrac{16}{20}} & {\text{ means that } 4 \cdot 20 = 16 \cdot x} \\ {\dfrac{5}{4} = \dfrac{x}{16}} & {\text{ means that } 5 \cdot 16 = 4 \cdot x} \\ {\dfrac{5}{4} = \dfrac{20}{x}} & {\text{ means that } 5 \cdot x = 4 \cdot 20} \end{array}\) 18 gramos de cobalto es a 10 gramos de plata ya que 36 gramos de cobalto es a 20 gramos de plata.
