Maths : Basic Numeracy

Q 112 / 370

UPSC CSE Prelims 2023

What is the number of selections of 10 consecutive things out of 12 things in a circle taken in the clockwise direction?

EXPLANATION

Correct Option (C)

The problem requires determining the number of selections of 10 consecutive items from 12 items arranged in a circle, with the selection proceeding in a clockwise direction.

When selecting 'r' consecutive elements from 'n' elements arranged in a circle, and a specific direction (such as clockwise) is stipulated, the number of distinct selections is 'n'. This is because each of the 'n' elements can serve as a unique starting point for a consecutive sequence of 'r' elements in the specified direction. Since n=12 and r=10, the number of such selections is 12.

Incorrect Options:

Options A (3), B (11), and D (66) are incorrect because they do not align with the combinatorial principle applicable to selecting consecutive items in a circular arrangement with a defined direction.

  • Option D (66) represents the number of combinations of selecting 10 items out of 12 without considering consecutiveness or circular arrangement, which is calculated as C(12, 10) or C(12, 2) = (12 × 11) / (2 × 1) = 66. This formula is not relevant when the items must be consecutive and selected in a specific direction from a circular arrangement.
  • Options A (3) and B (11) do not correspond to any standard combinatorial or permutation formula applicable to the given constraints.