My aunt uses an automobile sharing company hereafter abbreviated to ASC (eg: Autoshare or Zipcar), which permits only 200 km daily (excessive mileage is charged); but observe: ASC cares only about the final total mileage, and cares not about the distribution of the mileage over the number of days. Suppose that she rents for 3 days, for a maximum mileage of 600 km.
My aunt must travel from A to B, whose round-trip distance is confirmed as at least 400 km (per Google), but she cannot predict the precise distance (eg: detours, deviations due to road closures).
Objective: She needs to know the remaining mileage in real time (
= 600 - 400 ― 'Current distance travelled'), to decide whether enough mileage remains to drive elsewhere.
Problem: The ASC's odometer displays only 'Current distance travelled', and so requires my aunt to calculate manually (while driving) whenever she must know the remaining mileage.