One inch is equivalent to 25.4 mm. Knowing this information we can use the rule of three:
[tex]\begin{gathered} 25.4\operatorname{mm}\rightarrow1in \\ 80.81\operatorname{mm}\rightarrow x \end{gathered}[/tex]Where x is the diameter in inches and is given by:
[tex]\frac{80.81\operatorname{mm}\cdot1in}{25.4\operatorname{mm}}=3.18150\text{ inches}[/tex]So the diameter is 3.18150 inches.
