$\frac{a}{b} \times \frac{c}{d} = \frac{ac}{bd}$

a/b + c/d = (ad + bc)/bd

$\frac{a}{b} \div \frac{c}{d} = \frac{ad}{bc}$

$\frac{a}{b} - \frac{c}{d} = \frac{ad - bc}{bd}$