A bus runs between two towns at a steady 60 km/h and halts for a fixed total time at stops on the way, completing the trip in 4 hours. On another day it runs at 45 km/h with the same total halt time and takes 5 hours. Find the distance between the two towns.

Let the distance be $d$ km and the total halt time $h$ hours. $d/60+h=4$ and $d/45+h=5$. Subtracting: $d/45-d/60=1$, and since 1/45 - 1/60 = 1/180, this gives $d=180$, so $h=4-3=1$ hour. Check: 180/60 + 1 = 4 and 180/45 + 1 = 5. 240 km is $60\times 4$ and 225 km is $45\times 5$, both of which ignore the halts; the fact that those two answers disagree with each other shows the stopping time cannot be left out. 135 km is $45\times 3$.