If nC7 = nC20 find the value of n?
2025
If nC7 = nC20 find the value of n?
- A.
27
- B.
13
- C.
21
- D.
6
Attempted by 264 students.
Show answer & explanation
Correct answer: A
Key idea: use the symmetry property nCk = nC(n - k).
Set nC7 = nC20. By symmetry either 7 = 20 (impossible) or 7 = n - 20.
Solve 7 = n - 20 to get n = 27.
Check domain: n must be at least 20 for nC20 to be defined; 27 ≥ 20, so the solution is valid.
Answer: n = 27