[tex]20\%\text{ of } a=b \Longleftrightarrow0.2a=b\\\\b\%\text{ of } 20 \Longleftrightarrow \dfrac{b}{100}\cdot20\\\\\dfrac{b}{100}\cdot20=\dfrac{b}{5}=\dfrac{0.2a}{5}=\boxed{\boxed{a}}[/tex]
