$$\frac {a}{b} = \frac {c}{d}\Rightarrow a=\frac {b \times c}{d}$$
$$\frac {a}{b} = \frac {c}{d}\Rightarrow b=\frac {a \times d}{c}$$
$$\frac {a}{b} = \frac {c}{d}\Rightarrow c=\frac {a \times d}{b}$$
$$\frac {a}{b} = \frac {c}{d}\Rightarrow d=\frac {b \times c}{a}$$
