there are 10 meters in a decameter, DECA=10, and KILO=1000, 1000 meters to a kilimeter, thus there are 100 decameters in a kilometer
[tex]\bf \begin{array}{ccllll}
decameter&kilometer\\
\textendash\textendash\textendash\textendash\textendash\textendash&\textendash\textendash\textendash\textendash\textendash\textendash\\
100&1\\
42.3&x
\end{array}\implies \cfrac{100}{42.3}=\cfrac{1}{x}\implies x=\cfrac{42.3\cdot 1}{100}[/tex]
