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If 10 different balls are to be placed in 4 distinct boxes at random, then the probability that two of these boxes contain exactly 2 and 3 balls is :

JEE · Math · previous-year question

  1. A.$${{965} \over {{2^{11}}}}$$
  2. B.$${{965} \over {{2^{10}}}}$$
  3. C.$${{945} \over {{2^{11}}}}$$
  4. D.$${{945} \over {{2^{10}}}}$$correct

Answer

D. $${{945} \over {{2^{10}}}}$$

Explanation

Total ways of distribution = 410 = 220 Number of ways selecting two boxes out of four = 4C2 Then number of ways selecting 5 balls out of 10 = 10C5 Then no of ways of distributing 5 balls into two groups of 2 balls and 3 balls = 5C3.2! Then number of ways to distributing remaining balls into two boxes = 25 Number of ways placing exactly 2 and 3 balls in two of these boxes = 4C2 $$ \times $$ 10C5 $$ \times $$ 5C3.2! $$ \times $$ 25 = $${{{{6.252.10.2.2}^5}} \over {{2^{20}}}}$$ = $${{945} \over {{2^{10}}}}$$

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