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Question

If $^nC_3 = ^nC_8$, then find n.

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper (8-Mar-2021) (Shift 2)
The correct answer is
11

Combinations Property for nC3 = nC8

We are given the equation $^nC_3 = ^nC_8$.

Recall the property of combinations: $^nC_r = ^nC_k$ holds true if either:

  • $r = k$
  • $n = r + k$

In this problem, we have $r=3$ and $k=8$. Since $3 \neq 8$, we must use the second condition.

Finding the value of n

  1. Apply the condition $n = r + k$.
  2. Substitute the given values: $n = 3 + 8$.
  3. Calculate the sum: $n = 11$.

Therefore, the value of n is 11.

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