\displaystyle \frac{N1}{D1} \div \frac{N2}{D2} = {?}
Turn into a multiplication problem:
\displaystyle {} = \frac{N1}{D1} \times \frac{D2}{N2}
Reduce terms with common factors:
\displaystyle {} = \frac{SIMP_N1}{SIMP_D1} \times \frac{SIMP_D2}{SIMP_N2}
Combine into one fraction:
\displaystyle {} = \frac{SIMP_N1 \times SIMP_D2}{SIMP_D1 \times SIMP_N2}
Simplify:
\displaystyle {} = fractionReduce(N1* D2, D1 * N2)