GRE® Quant Practice | Circular Permutation

GRE Sample Questions | Circular Arrangement - With Constraint

This sample GRE quant practice question is a permutation combination problem solving question. Concept tested is rearranging objects around a circle with a constraint.

Question 8: 8 directors, the vice chairman and the chairman are to be seated around a circular table. If the chairman should sit between a director and the vice chairman, in how many ways can they be seated?

  1. 9!
  2. 7! × 2
  3. 9! × 2
  4. 8! × 2
  5. 8!

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Explanatory Answer | GRE Permuation Combination Question 8

Step 1: Possibilities where the chairman will be seated in between the vice chairman and a director

Essentially, the constraint boils down to seating the chairman and the vice chairman next to each other because there will definitely be a director on the other side of the chairman.
Consider the chairman and vice chairman as one unit.
The chairman and the vice chairman can be reordered within that unit in 2! ways.
We have to seat 8 directors and a unit comprising the chairman and vice chairman i.e., nine units around a circular table.

Step 2: Permute the group around a circular table after considering the seating constraint

n objects can be permuted around a table in (n - 1)! ways
9 people viz., 8 directors and a unit comprising chairman and vice chairman can be permuted around a circular table in (9 - 1)! = 8! ways.
Total number of outcomes = number of ways of reordering the chairman and vice chairman within the unit × number of ways permuting the group around the table.
= 8! × 2 ways

Choice D is the correct answer.

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