Dividing by a fraction is the same as multiplying by the reciprocal of the fraction.

The reciprocal of \dfrac{N2}{D2} is \dfrac{D2}{N2}.

\phantom{\dfrac{N1}{D1} \times \dfrac{D2}{N2}} = \dfrac{N1 \times D2}{D1 \times N2}

\phantom{\dfrac{N1}{D1} \times \dfrac{D2}{N2}} = \dfrac{NUMERATOR}{DENOMINATOR}