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Question

Set \(X\) contains \(3n\) elements and set \(Y\) contains \(2n\) elements, and they have \(n\) elements in common. How many elements does \((X – Y ) \times (Y – X)\) have?

This question was previously asked in
NDA 2 2025 GAT Question Paper (14-Sep-2025)
The correct answer is

\(2n^2\) 

To solve the problem, we need to determine the number of elements in the product set \((X - Y) \times (Y - X)\), where \(X\) and \(Y\) are given sets with specific properties:

  1. Set \(X\) contains \(3n\) elements.
  2. Set \(Y\) contains \(2n\) elements.
  3. They have \(n\) elements in common.

First, let's determine the elements in each set that are not in the other:

  • \(X - Y\): The elements in \(X\) that are not in \(Y\).
  • \(Y - X\): The elements in \(Y\) that are not in \(X\).

Since \(X\) and \(Y\) have \(n\) elements in common, we can calculate the number of elements in each of these differences:

  • \(X - Y\) contains \(3n - n = 2n\) elements.
  • \(Y - X\) contains \(2n - n = n\) elements.

Next, we find the Cartesian product \((X - Y) \times (Y - X)\). This product consists of ordered pairs \((a, b)\) where \(a\) is an element of \(X - Y\) and \(b\) is an element of \(Y - X\).

The number of such ordered pairs is equal to the product of the number of elements in \(X - Y\) and \(Y - X\):

\(| X - Y | \times | Y - X | = 2n \times n = 2n^2\)

Therefore, the number of elements in the set \((X - Y) \times (Y - X)\) is \(2n^2\).

Thus, the correct answer is:

\(2n^2\).

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