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Q79·CSAT · Prelims 2023

Painted cuboid cut into 1cm cubes

ReasoningCube & Painted FacesCube & geometryMedium

Question

A cuboid of dimensions 7 cm × 5 cm × 3 cm is painted red, green and blue colour on each pair of opposite faces of dimensions 7 cm × 5 cm, 5 cm × 3 cm, 7 cm × 3 cm respectively. Then the cuboid is cut and separated into various cubes each of side length 1 cm. Which of the following statements is/are correct?

1There are exactly 15 small cubes with no paint on any face.
2There are exactly 6 small cubes with exactly two faces, one painted with blue and the other with green.

Options

a

1 only

Answer
b

2 only

c

Both 1 and 2

d

Neither 1 nor 2

Explanation

Evaluate each structural block statement independently [cite: 4438, 4439, 4491, 4492]:

Statement 1 tracks cubes with no paint, which forms the interior unexposed core block[cite: 4444, 4496]. Reduce each dimension of the cuboid by 2 cm to strip away the outer face layer: Core Volume = (7 - 2) × (5 - 2) × (3 - 2) = 5 × 3 × 1 = 15 cubes. [cite: 4444, 4496] Statement 1 is exactly correct .

Statement 2 tracks edge cubes with exactly two painted faces: one blue and one green[cite: 4445, 4497]. Examine the dimension mapping: Blue paint is on the 7 × 3 faces, and Green paint is on the 5 × 3 faces[cite: 4438, 4491]. These specific faces meet along the edges that share the dimension of 3 cm. A standard cuboid features exactly 4 parallel edges matching this orientation. The number of 2-faced cubes residing strictly along any edge of length L is calculated as (L - 2). For our 3 cm edge: 3 - 2 = 1 cube per edge. Total blue-green cubes = 4 edges × 1 cube/edge = 4 cubes. Since the statement asserts there are 6, Statement 2 is incorrect[cite: 4445, 4497, 4521].

The volume of completely unpainted internal elements inside an X × Y × Z cut cuboid is universally calculated using the reduced formula (X-2)(Y-2)(Z-2).

Answer: (a).

Question details

Year

2023

Paper

CSAT

Question

Q79

Section

Logical & Analytical Reasoning

Sub-topic

Cube & Painted Faces

Type

Cube & geometry

Difficulty

Medium

Source hint

Spatial reasoning

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