Reasoning : General Mental Ability

Q 37 / 45

UPSC CSE Prelims 2013

A cube has six numbers marked 1, 2, 3, 4, 5 and 6 on its faces. Three views of the cube are shown below:

Figure for the question: A cube has six numbers marked 1, 2, 3, 4, 5 and 6 on its faces. Three views of the cube a…

What possible numbers can exist on the two faces marked (A) and (B), respectively on the cube?

Figure for the question: A cube has six numbers marked 1, 2, 3, 4, 5 and 6 on its faces. Three views of the cube a…

EXPLANATION

Correct Option

The numbers 2 and 3 can exist on faces (A) and (B), respectively.

Based on the initial three views of the cube, the opposite faces can be determined through logical deduction:

  • Comparing View 1 (showing faces 1, 2, 3) and View 3 (showing faces 1, 5, 4), with '1' as the common face, a systematic rotation reveals the following relationships:
    • Face 2 is opposite Face 5.
    • Face 3 is opposite Face 4.
  • Consequently, the remaining two numbers, 1 and 6, must be opposite each other.

Thus, the established pairs of opposite faces are: (1, 6), (2, 5), and (3, 4).

In the target view, the visible faces are 6, (A), and (B). A fundamental principle of a standard cube is that opposite faces cannot be simultaneously visible in any single perspective.

  • Since face 6 is visible, its opposite face, 1, cannot be present on either face (A) or face (B).
  • The remaining available numbers for faces (A) and (B) are 2, 3, 4, and 5 (excluding 1 and 6).
  • Additionally, faces (A) and (B) must not be opposite to each other.
  • If face (A) is 2 and face (B) is 3:
    • Neither 2 nor 3 is opposite to 6.
    • Faces 2 and 3 are not opposite to each other (2 is opposite 5, and 3 is opposite 4).
    • This combination is consistent with all established rules and relationships for a standard cube.

Incorrect Options

  • Option 2 (6 and 1): This option is incorrect. Firstly, the number 6 is already visible on one face of the cube; therefore, face (A) cannot also be 6. Secondly, 6 and 1 are opposite faces, and opposite faces cannot be simultaneously visible on a single view of the cube.

  • Option 3 (1 and 4): This option is incorrect because 6 and 1 are opposite faces. Since 6 is already visible in the given view, its opposite face, 1, cannot be visible on face (A).

  • Option 4 (3 and 1): This option is incorrect because 6 and 1 are opposite faces. Since 6 is already visible in the given view, its opposite face, 1, cannot be visible on face (B).

Explanation figure for the question: A cube has six numbers marked 1, 2, 3, 4, 5 and 6 on its faces. Three views of the cube a…