UPSC Prelims 2026 · CSAT Paper 2 · Set DMathematicsQ 31 / 80

UPSC Prelims 2026 CSAT Paper 2 — Question 31

Question

Seven cubes are identical in shape. Out of these, the weight of each of the six cubes is equal and the weight of the remaining cube is less than the weight of any other cube. A balance is used to identify the lightest cube. What is the minimum number of attempts required to distinguish the odd cube with certainty?

(a)2
(b)3
(c)4
(d)1

UnlockIAS answer: (a)2

Solution

Split 7 cubes as 3-3-1. First weigh 3 vs 3: if balanced, the odd one is the spare; if not, it is in the lighter pan of three. A second weighing (1 vs 1 of the suspect three) isolates it. So 2 attempts suffice.

UnlockIAS expert key, prepared 24 May 2026 and cross-verified by re-deriving the answer — this is our key, not the official UPSC provisional key (awaited).

Section

Mathematics

Topic Reference

Logical Reasoning — Weighing puzzle

Official UPSC key still awaited

UPSC has announced (PIB Release 2262441, 18 May 2026) that the provisional answer key for CSE Prelims 2026 will be released “soon after Exam”. Until then, use the CSAT score calculator for your qualifying-status estimate, and join the per-question discussion below.

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Official Notification

PIB Release 2262441

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