Answer:

Step-by-step explanation:
In the figure attached, map shows the dimensions of the 5 counties in a Midwestern state.
Adams county has the triangular boundaries.
Since area of triangle is represented by the expression 
Base represents the boundary of county Harding touching Adams = 18 mi.
Height represents the boundary of county Lincoln touching Adams = 10 mi
Therefore, expression representing the area of Adams county will be

= 
= 90 square miles.