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Permutations and Combinations question

2012 · Shift 1 · Q25
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  5. /2012 · Shift 1 · Q25

Permutations and Combinations question

2012 · Shift 1 · Q25

JEE AdvancedMathematicsPermutations and CombinationsMCQ+4 / −1
The total number of ways in which 5 balls of different colours can be distributed among 3 persons so that each person gets at least one ball is
  1. A
    75
  2. B
    150
  3. C
    210
  4. D
    243
View written solutionFree

Correct answer: B

  1. Interpret the problem

We have:

  • 555 balls, all of different colours  so they are distinct.
  • 333 persons, also distinct.
  • Each person must get at least one ball.

We need the number of onto distributions of 555 distinct objects among 333 distinct persons.


  1. Total distributions without restriction

Each of the 555 distinct balls can be given to any one of the 333 persons.

So total number of distributions is 35=243.3^5 = 243.35=243.


  1. Subtract distributions where at least one person gets no ball

Let the three persons be P1,P2,P3P_1, P_2, P_3P1​,P2​,P3​.

Define:

  • AiA_iAi​ = set of distributions where person PiP_iPi​ gets no ball.

If one specific person gets no ball, then each ball can go to only the remaining 222 persons. Thus, ∣Ai∣=25=32.|A_i| = 2^5 = 32.∣Ai​∣=25=32.

Since there are 333 persons, ∣A1∣+∣A2∣+∣A3∣=3⋅25=96.|A_1|+|A_2|+|A_3| = 3 \cdot 2^5 = 96.∣A1​∣+∣A2​∣+∣A3​∣=3⋅25=96.


  1. Add back distributions where two persons get no ball

If two specific persons get no ball, then all 555 balls must go to the remaining one person. So for any pair, ∣Ai∩Aj∣=1.|A_i \cap A_j| = 1.∣Ai​∩Aj​∣=1.

Number of such pairs is (32)=3\binom{3}{2}=3(23​)=3. Hence, ∣A1∩A2∣+∣A2∩A3∣+∣A3∩A1∣=3.|A_1\cap A_2|+|A_2\cap A_3|+|A_3\cap A_1| = 3.∣A1​∩A2​∣+∣A2​∩A3​∣+∣A3​∩A1​∣=3.

There is no case where all three get no ball.


  1. Apply Inclusion-Exclusion Principle

Required number of distributions: 35−(31)25+(32)153^5 - \binom{3}{1}2^5 + \binom{3}{2}1^535−(13​)25+(23​)15 =243−96+3= 243 - 96 + 3=243−96+3 =150.= 150.=150.


  1. Check with options

The correct option is: 150\boxed{150}150​ which is Option B.


  1. Comparison with stored correct answer

Stored correct answer: B

Our derived answer: B

So they agree.

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