Respuesta :

[tex]\bf \textit{circumference of a circle}\\\\ C=2\pi r\qquad \begin{cases} diameter=d\\ r=radius=\frac{d}{2}\\ ----------\\ C=78.5\\ \end{cases} \\\\\\ 78.5=2\pi r\implies 78.5=2\pi \cdot \cfrac{d}{2}[/tex]

solve for "d"