Question:

\( S = \{1,2,3,\dots,20\} \) is to be partitioned into four sets \( A, B, C, D \) of equal size. The number of ways is

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Partition into equal labeled sets → divide by factorial of group sizes only.
Updated On: Apr 23, 2026
  • $\dfrac{20!}{4!\cdot 5!}$
  • $\dfrac{20!}{4^5}$
  • $\dfrac{20!}{(5!)^4}$
  • $\dfrac{20!}{(4!)^5}$
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The Correct Option is C

Solution and Explanation

Concept: Partition of set into equal groups

Step 1:
Total elements = 20, each set has 5 elements.

Step 2:
Arrange all elements: 20!

Step 3:
Divide by $(5!)^4$ (internal arrangements of each set).

Step 4:
Since sets are distinct (A, B, C, D), no further division.
Conclusion:
Number of ways = $\dfrac{20!}{(5!)^4}$
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