we assume the tower is constructed by stacking one block on top of another, hence the width, length and height of each 'floor' of the tower are 2.5 inches... we further assume that the question is asking about volume rather than surface area or other possibilities

for a square/rectangle:

volume = length * width * height

total volume is then five times the volume of a single block:

5 * 2.5^3 = 31.25

if our assumptions are correct, the tower has a total volume of 31.25 in^3