\( \Large \frac{d}{4} - \frac{d}{5} = \frac{7\frac{1}{2}}{60} \)

\( \Large \frac{5d - 4d}{20} = \frac{15}{120} = \frac{1}8{}\)

\( \Large \frac{d}{20} = \frac{1}{8}\)

D = 2.5 km.