there are 100 cm in 1 meter, and there are 1000 meters in 1 Kilometer.
so in 18 Kilometers there are 18*1000 meters, and there are 18*1000*100 centimeters.
[tex] \bf \cfrac{map}{actual}~\hspace{7em}\stackrel{\textit{both sides in cm}}{\cfrac{15}{1800000}}\implies \stackrel{simplified}{\cfrac{1}{120000}}\qquad 1:120000 [/tex]