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NDA 2025
Exam
NDA
Year
2025
Subject
Mathematics
Chapter
Permutation and Combination

NDA 2025 · Mathematics · Permutation and Combination

Let y = x! and z = (2x)! . If (z/y) = 120 , then what is the value...

Let y=x!y = x! and z=(2x)!z = (2x)!. If (z/y)=120(z/y) = 120, then what is the value of (3x)!(3x)!?

Options

  1. A362880362880
  2. B181440181440
  3. C9072090720
  4. D4536045360

Correct answer

A. 362880362880

Step-by-step solution

Given y=x!y = x! and z=(2x)!z = (2x)!.

We are given zy=120\dfrac{z}{y} = 120.

Substituting the values of yy and zz, we get:

(2x)!x!=120\dfrac{(2x)!}{x!} = 120

Checking for small integer values of xx:

For x=1x = 1, 2!1!=2\dfrac{2!}{1!} = 2

For x=2x = 2, 4!2!=242=12\dfrac{4!}{2!} = \dfrac{24}{2} = 12

For x=3x = 3, 6!3!=7206=120\dfrac{6!}{3!} = \dfrac{720}{6} = 120

Thus, x=3x = 3.

We need to find the value of (3x)!(3x)!.

Substituting x=3x = 3, we get (3×3)!=9!(3 \times 3)! = 9!.

9!=9×8×7×6×5×4×3×2×1=3628809! = 9 \times 8 \times 7 \times 6 \times 5 \times 4 \times 3 \times 2 \times 1 = 362880.

Answer: 362880362880
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