Math Help - Metric of cities

1. Metric of cities

Let $X$ be the set of cities, $a,b,c.$ Indicate which metric on $X$ must consider a traveller, if he moves in: a car, a plane, a train and a bike.

I don't get well the problem. How to proceed?

2. Originally Posted by Connected
Let $X$ be the set of cities, $a,b,c.$ Indicate which metric on $X$ must consider a traveller, if he moves in: a car, a plane, a train and a bike.

I don't get well the problem. How to proceed?
This makes zero, absolutely no sense haha. Did you copy it wrong?

3. No, I copied it well.

Haha, for me doesn't make any sense too!