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

Answer: Option A

$$\begin{array}{rl}& 20\%\text{of}a=b\Rightarrow \frac{20}{100}a=b\\ & \therefore b\%\text{of}20=\frac{b}{100}\times 20\\ & =\frac{20}{100}a\times \frac{1}{100}\times 20\\ & =\frac{4}{100}a=4\%\text{of }a\end{array}$$