You may have correctly converted the distance between cities to 921 kilometers, and then solved the proportion $\displaystyle\frac{{307}}{{n}}=\frac{{921}}{{1}}\usepackage{cmbright}\Huge\$ . The correct equation would be $\displaystyle\frac{{{921}\ \text{ km}}}{{n}}=\frac{{{307}\ \text{ km}}}{{{1}\ \text{ hr}}}\usepackage{cmbright}\Huge\$ .