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Probability question

2020 · 9 Jan · Shift 2 · Q21
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Probability question

2020 · 9 Jan · Shift 2 · Q21

JEE MainMathematicsProbabilityMCQ+4 / −1
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 :
  1. A
    965211{{965} \over {{2^{11}}}}211965​
  2. B
    965210{{965} \over {{2^{10}}}}210965​
  3. C
    945211{{945} \over {{2^{11}}}}211945​
  4. D
    945210{{945} \over {{2^{10}}}}210945​
View written solutionFree

Correct answer: D

  1. Total number of ways

Each of the 101010 different balls can go into any of the 444 distinct boxes.

Therefore, total number of possible distributions is 410.4^{10}.410.


  1. Favourable cases

We want exactly two boxes to contain exactly 222 and 333 balls.

This means:

  • one box has 222 balls,
  • another box has 333 balls,
  • the remaining 555 balls go into the other two boxes.

Also, since the statement says two of these boxes contain exactly 2 and 3 balls, the remaining two boxes must contain numbers of balls different from 222 and 333.


  1. Choose the boxes for 2 and 3 balls

Choose the box that gets 222 balls and the box that gets 333 balls: 4P2=4×3=12.^4P_2 = 4\times 3 = 12.4P2​=4×3=12.


  1. Choose the balls for these boxes
  • Choose 222 balls out of 101010 for the first chosen box: (102).\binom{10}{2}.(210​).
  • Choose 333 balls out of remaining 888 for the second chosen box: (83).\binom{8}{3}.(38​).

So far, number of ways: 12(102)(83).12\binom{10}{2}\binom{8}{3}.12(210​)(38​).

After this, 555 balls remain to be distributed among the remaining 222 boxes.


  1. Distribute remaining 5 balls into 2 boxes

Each of the 555 remaining distinct balls can go into either of the 222 remaining boxes, so total distributions: 25=32.2^5 = 32.25=32.

But we must exclude cases where one of these two boxes also gets exactly 222 or 333 balls.

Possible splits of 555 into two boxes are: (0,5),(1,4),(2,3),(3,2),(4,1),(5,0).(0,5),(1,4),(2,3),(3,2),(4,1),(5,0).(0,5),(1,4),(2,3),(3,2),(4,1),(5,0).

Invalid splits are (2,3)(2,3)(2,3) and (3,2)(3,2)(3,2), because then the remaining boxes also contain exactly 222 and 333 balls, giving more than the intended pair.

Number of ways corresponding to split (2,3)(2,3)(2,3) or (3,2)(3,2)(3,2): (52)+(53)=10+10=20.\binom{5}{2}+\binom{5}{3}=10+10=20.(25​)+(35​)=10+10=20.

Hence valid distributions for remaining 555 balls: 25−20=32−20=12.2^5-20=32-20=12.25−20=32−20=12.


  1. Total favourable ways

Thus, Nf=12(102)(83)×12.N_f=12\binom{10}{2}\binom{8}{3}\times 12.Nf​=12(210​)(38​)×12.

Now, (102)=45,(83)=56.\binom{10}{2}=45,\qquad \binom{8}{3}=56.(210​)=45,(38​)=56.

So, Nf=12×45×56×12.N_f=12\times 45\times 56\times 12.Nf​=12×45×56×12.

Compute: 45×56=2520,45\times 56=2520,45×56=2520, 12×12=144,12\times 12=144,12×12=144, therefore Nf=2520×144=362880.N_f=2520\times 144=362880.Nf​=2520×144=362880.


  1. Probability

P=362880410.P=\frac{362880}{4^{10}}.P=410362880​.

Since 410=(22)10=220=1048576,4^{10}=(2^2)^{10}=2^{20}=1048576,410=(22)10=220=1048576, we get P=3628801048576.P=\frac{362880}{1048576}.P=1048576362880​.

Simplify: 362880=945×384,362880=945\times 384,362880=945×384, and 1048576=210×1024=1024×1024.1048576=2^{10}\times 1024=1024\times 1024.1048576=210×1024=1024×1024.

More directly, 3628801048576=945210.\frac{362880}{1048576}=\frac{945}{2^{10}}.1048576362880​=210945​.

Hence, P=945210.P=\frac{945}{2^{10}}.P=210945​.


  1. Check with options

This matches Option D: 945210.\boxed{\frac{945}{2^{10}}}.210945​​.


  1. Comparison with stored answer

Stored correct answer: D

Our derived answer: D

So the stored answer is correct.

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