The ratio between the map and the real distance is given at first: 1 inch to 10.4 miles.
If we want to know the real distance that is represented as 1.5 inches in the map, we can use the rule of three:
1 inch --- 10.4 miles
1.5 inches --- x miles
So, we can put an equal sign to get:
[tex]\begin{gathered} \frac{1}{1.5}=\frac{10.4}{x} \\ 1\cdot x=1.5\cdot10.4 \\ x=15.6 \end{gathered}[/tex]So, the two cities are 15.6 miles apart.