UPSC Prelims 2019 CSAT Paper II · Q72 of 79 Mental Ability medium

Each face of a cube can be painted in black or white colours. In how many different ways can the cube be painted ?

  1. (a) 9
  2. (b) 10 ✓ UPSC's answer
  3. (c) 11
  4. (d) 12

Why the answer is (b)

• The problem asks for the number of distinct colorings of a cube's faces using two colors (black and white), where rotations of the cube are considered identical.

• We classify the distinct cubes by the number of black faces (k), ranging from 0 to 6.

• For k=0 (0 black, 6 white) and k=6 (6 black, 0 white), there is exactly 1 distinct way each.

• For k=1 (1 black, 5 white) and k=5 (5 black, 1 white), the single colored face can be in any position, but all positions are rotationally equivalent, so there is 1 distinct way for each.

• For k=2 (2 black, 4 white), the two black faces can be either adjacent or opposite, resulting in 2 distinct ways.

• For k=4 (4 black, 2 white), the two white faces can be either adjacent or opposite, resulting in 2 distinct ways.

• For k=3 (3 black, 3 white), the three black faces can meet at a single corner (forming an L-shape) or form a 'U' shape (two opposite and one adjacent to both), resulting in 2 distinct ways.

• Summing these cases: 1 (k=0) + 1 (k=1) + 2 (k=2) + 2 (k=3) + 2 (k=4) + 1 (k=5) + 1 (k=6) = 10 distinct ways.

Why the other options are wrong

(a) 9
Option (a) is incorrect because it undercounts the distinct configurations, specifically missing the multiple arrangements possible for 2, 3, or 4 black faces.
(c) 11
Option (c) is incorrect because it overcounts the distinct configurations, likely by treating rotationally equivalent arrangements as distinct or miscounting the 3-face case.
(d) 12
Option (d) is incorrect because it overcounts the distinct configurations, failing to account for the symmetries that reduce the total number of unique patterns to 10.

Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2019, held on 2 June 2019. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.

Reading the answer is not the same as getting it right under a clock. Practise this question with UPSC's negative marking, and anything you miss goes into an error notebook until you get it right twice.

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