Algebra Rule 10
If the numerator and denominator of a fraction are both fractions, it can be converted into a fraction of two multiplications
```{\left({a \over c}\right) \over \left({b \over d}\right)} = {ad \over bc}```
Description:
Combining rules 3 and 4, we can multiply the denominator of the bottom fraction with the numerator of the upper fraction, which gives the combined numerator, and cancels the denominator of the lower fraction; we can then multiply the denominator of the upper fraction with the numerator of the lower fraction, to give the combined denominator and cancel the denominator of the upper fraction.
```{\left({4 \over 5}\right) \over \left({2\over1}\right)} = {0.8 \over 2} = {4 \over 10} = {4*1 \over 2*5}```