If the scale on the map is 1 inch=20 km then if two points are separated by 20 km in real life then they will be separated by 1 inch on the map. If we want to know the distance on the map for two cities separated by 170 km in real life we can use the rule of three:
![\begin{gathered} 20\operatorname{km}\rightarrow1in \\ 170\operatorname{km}\rightarrow x \end{gathered}]()
Where x is given by:
![x=\frac{170\operatorname{km}\cdot1in}{20\operatorname{km}}=8.5in]()
Then the answer is 8.5 inches.