we know that
the circumference of a circle is equal to
[tex]C=2\pi r[/tex]
where
r is the radius of the circle
in this problem we have
[tex]C=84.78\ mm\\r=?\\\pi=3.14[/tex]
Solve for r
[tex]C=2\pi r[/tex]
[tex]r=\frac{C}{2\pi}[/tex]
substitute the values in the formula
[tex]r=\frac{84.78}{2*3.14}[/tex]
[tex]r=13.5\ mm[/tex]
therefore
the answer is
the radius of the brazilian coin is [tex]13.5\ mm[/tex]
