If 20% of a = b, then b% of 20 is the same as:

$$\eqalign{ & 20\% \,{\text{of}}\,a = b \Rightarrow \frac{{20}}{{100}}a = b \cr & \therefore b\% \,{\text{of}}\,20 = {\frac{b}{{100}} \times 20} \cr & = {\frac{{20}}{{100}}a \times \frac{1}{{100}} \times 20} \cr & = \frac{4}{{100}}a = 4\% \,\,{\text{of }}\,a \cr} $$