Express the following rational number in standard form:

$\dfrac{-12}{30}$

A rational number $\dfrac{p}{q}$ is in standard form if $q \gt 0$ and $p, q$ have no common divisor other than 1.

Here the denominator 30 is already positive.

The greatest common divisor of 12 and 30 is 6.

Now,

$\dfrac{-12}{30} =\dfrac{-12 \div 6}{30 \div 6} = \dfrac{-2}{5}$

Hence, the standard form of $\mathbf{\dfrac{-12}{30} \text{ is } \dfrac{-2}{5}}$.