Problem

The key is to unfold the cube into a flat net so that the shortest path becomes a straight line. Since the ant must travel on the cube's surface, the farthest corner is diagonally opposite—but the actual shortest route isn't through the interior. By unfolding two adjacent faces, you can see that the straight-line distance on the net is shorter than going along edges, and the minimum time is simply that distance divided by speed $u$.

The key is to unfold the cube into a flat net so that the shortest path becomes a straight line. Since the ant must travel on the cube's surface, the farthest corner is diagonally opposite—but the actual shortest route isn't through the interior. By unfolding two adjacent faces, you can see that the straight-line distance on the net is shorter than going along edges, and the minimum time is simply that distance divided by speed $u$.