An army $40$ km long is moving at constant speed. A horseman takes a message from the rear end of the army to the general in the front end of the army and returns back to the rear end of the army (moving with constant speed). During this time, the army moves a total of $40$ km.
What total distance did the horseman travel?
To me, we can draw place-time $(x,t)$-plane.
The sketch might help if velocity $V$ is taken properly. Note the horseman's track. (Sketch would be moved to answers area later.)