A very simple rule:
Whenever you a dividing a number (or fraction) by a fraction, you simply multiply the number (or fraction) by the reciprocal of the dividing fraction.
As an example, let's take a look at dividing a fraction by another fraction:
[tex]\frac{2}{5}\div\frac{2}{3}[/tex]This division can be performed by multiplying 2/5th by the reciprocal of (2/3)rd, which is 3/2. Thus, it can be performed by:
[tex]\begin{gathered} \frac{2}{5}\div\frac{2}{3} \\ =\frac{2}{5}\times\frac{3}{2} \\ =\frac{2\times3}{5\times2} \\ =\frac{6}{10} \\ =\frac{3}{5} \end{gathered}[/tex]