Let's convert everyone of them into meters:
[tex]\begin{gathered} 4.8\operatorname{km}\times\frac{1000m}{1\operatorname{km}}=4800m \\ 15.840ft\times\frac{1m}{3.281ft}=4.828m \end{gathered}[/tex]Since:
[tex]4800>4.828[/tex]4.8 kilometers is larger than 15.840 feet
