Theory of Intelligence
11TH•Mathematics•Permutations And Combinations•Question 7

Overview

There are four distinct colored balls and four boxes of same colors as those of the balls. Determine the number of possible ways the balls, one each in a box, can be placed such that a ball does not go to a box of its own colour.

This is the number of derangements of 4 distinct objects:

[ !4 = 4!\left(1 - 1 + \dfrac12 - \dfrac16 + \dfrac1{24}\right) = 24\left(\dfrac12 - \dfrac16 + \dfrac1{24}\right) = 24\cdot\dfrac{12-4+1}{24} = 9. ]

(Alternatively: total permutations ( 4! = 24 ), subtract those with at least one fixed point using inclusion-exclusion yields 9.)

Answer: 9 ways

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