Answer:
9 inches.
Step-by-step explanation:
Fun question! The length of a space diagonal for a cube is
Thus, the first cube has a side diagonal of
The next cube would have a side diagonal of 3
The third cube would have a side diagonal of
And the fourth cube would have a side diagonal of 9.
Since the fifth cube has a side length equal to the side diagonal of the fourth cube, it should be 9 inches.