for the first one
[tex]\begin{gathered} C=\pi\times4 \\ C=12.56 \end{gathered}[/tex]for the second one
[tex]\begin{gathered} C=(3.14)\times20 \\ C=62.8 \end{gathered}[/tex]and the last one
[tex]\begin{gathered} 28.26=(3.14)\times d \\ \frac{28.26}{3.14}=\frac{3.14\times d}{3.14} \\ d=9 \end{gathered}[/tex]