Question

Erin the ant starts at a given corner of a cube and crawls along exactly 7 edges in such a way that she visits every corner exactly once and then finds that she is unable to return along an edge to her starting point. How many paths are there meeting these conditions?

$6$

$9$

$12$

$18$

$24$

Solution:

The first two edges can be chosen in $3\cdot2=6$ ways. These two edges determine an initial face of the cube. After those moves, there is one unvisited vertex on that initial face.

That remaining vertex must be visited next; otherwise Erin would later reach it after all of its neighbors had already been visited, and the path could not continue. The last four vertices are then on the opposite face and can be visited in two cyclic orders.

Of those two orders, exactly one ends at a vertex not adjacent to the starting point. Hence there are $6$ valid paths.

Thus, the correct answer is A.

2015 AMC 10B Problem 20

Solution:

Problem 20 in Other Years