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Permutations and Combinations question

2025 · 2 Apr · Shift 2 · Q28
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Permutations and Combinations question

2025 · 2 Apr · Shift 2 · Q28

JEE MainMathematicsPermutations and CombinationsMCQ+4 / −1
The number of ways, in which the letters A, B, C, D, E can be placed in the 8 boxes of the figure below so that no row remains empty and at most one letter can be placed in a box, is : JEE Main 2025 (Online) 2nd April Evening Shift Mathematics - Permutations and Combinations Question 6 English
  1. A
    5880
  2. B
    840
  3. C
    960
  4. D
    5760
View written solutionFree

Correct answer: D

Let the figure have 3 rows with boxes distributed as shown in the standard pattern for this question:

  • Top row: 222 boxes
  • Middle row: 333 boxes
  • Bottom row: 333 boxes

So total boxes =2+3+3=8=2+3+3=8=2+3+3=8.

We must place the 5 distinct letters A,B,C,D,EA,B,C,D,EA,B,C,D,E into these 8 boxes such that:

  1. At most one letter is placed in a box.
  2. No row remains empty.

1. Total unrestricted placements

Choose 5 of the 8 boxes and arrange the 5 distinct letters in them:

8P5=8!3!=6720{}^8P_5 = \frac{8!}{3!}=67208P5​=3!8!​=6720

This counts all placements, including those where one or more rows may be empty.


2. Subtract arrangements where some row is empty

We use inclusion-exclusion.

Let:

  • E1E_1E1​ = top row empty
  • E2E_2E2​ = middle row empty
  • E3E_3E3​ = bottom row empty

We count each.

Case 1: Top row empty

Then all 5 letters must be placed in the remaining 666 boxes.

Number of ways:

6P5=6⋅5⋅4⋅3⋅2=720{}^6P_5 = 6\cdot5\cdot4\cdot3\cdot2 = 7206P5​=6⋅5⋅4⋅3⋅2=720

Case 2: Middle row empty

Then only top + bottom rows are available: 2+3=52+3=52+3=5 boxes. All 5 letters must occupy all 5 boxes.

Number of ways:

5!=1205! = 1205!=120

Case 3: Bottom row empty

Similarly, available boxes = 2+3=52+3=52+3=5.

Number of ways:

5!=1205! = 1205!=120

So total to subtract initially:

720+120+120=960720+120+120=960720+120+120=960


3. Check pairwise intersections

If two rows are empty, all 5 letters would have to be placed in the boxes of the remaining single row. But the rows have sizes 2,3,32,3,32,3,3, none of which can hold 5 letters.

Hence,

E1∩E2=E2∩E3=E3∩E1=∅E_1\cap E_2 = E_2\cap E_3 = E_3\cap E_1 = \varnothingE1​∩E2​=E2​∩E3​=E3​∩E1​=∅

and triple intersection is also impossible.

So no further correction is needed.


4. Final count

Therefore, required number of ways:

6720−960=57606720-960=57606720−960=5760


5. Option check

  • A: 588058805880 ❌
  • B: 840840840 ❌
  • C: 960960960 ❌
  • D: 576057605760 ✅

So the correct answer is:

5760\boxed{5760}5760​

Hence, Option D is correct.

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