A goods lorry is scheduled to cover 240 km in 4 hours but leaves the depot 48 minutes late. To reach at the scheduled time it must run at a speed (in km/h) of:

The schedule allows 4 hours at 240/4 = 60 km/h; the late start leaves 4 − 0.8 = 3.2 hours, so the lorry needs 240/3.2 = 75 km/h. (The time falls by 20%, so the speed must rise by 25%, not by 20% — raising 60 by 20% to 72 km/h would still bring the lorry in late. 80 km/h assumes a full hour was lost rather than 48 minutes, and 60 is the speed the schedule was built on.)