\( \Large \frac{d}{4} - \frac{d}{5} \) = \(  \Large \frac{15}{60} \)

= \( \Large \frac{5d - 4d}{20} \) = \(  \Large \frac{15}{60} \)

= \( \Large \frac{d}{20}\) = \(  \Large \frac{1}{4} \)

=> d = 5 km