If $a$ is the reciprocal of $b$, then $a\times b$ equals

For what value of $x$, do the rational numbers $\frac{x}{28}$ and $\frac{3}{14}$ become equivalent ?

For what value of $x$, do the rational numbers $\frac{-18}{5}$ and $\frac{x}{10}$ become equivalent ?

For what value of $x$, do the rational numbers $\frac{15}{x}$ and $\frac{-3}{8}$ become equivalent ?

Write the rational number $\frac{8}{-36}$ in its standard form.

Write the rational number $\frac{-7}{-91}$ in its standard form.