Answer
$255\;km$.
Work Step by Step
The distance on the map measures $a=3\;cm$.
Let the actual distance is $=d$.
Equate the scale drawing ratio with $a:d$.
$\Rightarrow \frac{1}{85}=\frac{3}{d}$
Cross multiply.
$\Rightarrow 1d=85(3)$
Simplify.
$\Rightarrow d=255\;km$.
