An ant wants to travel from one corner of the floor of a cubical room to the diagonally opposite corner of the ceiling. The length of 1 side of the room is 10 ft. What is the shortest possible distance travelled by the ant?

The shortest distance possible would be from the corner of the floor to the midpoint of the other vertical edge of the adjacent wall to the corner of the ceiling. If you open the 2 walls along this edge, the path travelled by the ant becomes the diagonal of a rectangle with its length equal to 20 ft and breadth = 10 ft and the diagonal is the shortest possible distance between 2 opposite vertices of a rectangle. Length of this diagonal is 10 x 5^0.5 which is choice 3.