9) To glue together the faces of two unit cubes, 0.25 grams of glue are required. How much glue is needed to build a $5 \times 5 \times 5$ cube from 125 unit cubes? (N.B. to ensure greater stability, all pairs of faces in contact are glued)
(A) $180 \mathrm{~g}$
(B) $150 \mathrm{~g}$
(C) $90 \mathrm{~g}$
(D) $75 \mathrm{~g}$
(E) $125 \mathrm{~g}$.
9) The answer is $(D)$

The total number of faces is 900, and the non-glued faces are only the external ones, whose number is equal to the total surface area of the cube, which is 150. Therefore, there are 375 pairs of faces to glue, and thus 75 grams of glue are required.