Two taxi firms work the road between the town and the airport, and a local newspaper spent a whole column comparing them last spring.
The first firm charges 5,000 som to come to your door, then 1,200 som for each kilometre. The second charges 10,000 som to come out, then — and this is the part that surprised the paper's readers — also 1,200 som a kilometre.
Everybody assumed there would be a distance at which the second firm became the better choice. There is not. For a ten-kilometre trip the first firm asks 17,000 som and the second asks 22,000. For a fifty-kilometre trip the first asks 65,000 and the second asks 70,000. The difference is 5,000 som at ten kilometres and 5,000 som at fifty, because the two firms charge the same for every kilometre travelled: only the call-out fee differs, and a fee you pay once does not shrink with distance.
Drawn on a chart, the two fares are two straight lines climbing at exactly the same steepness, one always five thousand som above the other. The gap stays the same, so the lower line never catches up with the upper one, and there is no crossing point.
The column ended with a sentence the editor liked enough to put in bold: a cheaper rate can be caught up with, but a cheaper starting fee, at the same rate, never can.