We need to find n such that choosing 3 people from that where order doesn't matter and the number of ways is greater than 365. We use the formula for combination:
ncr = (n!)/r!(n-r)! In this case we will --> (n!)/3!(n-3)!
choose 3 people
The number of ways for this must be greater than 365:
(n!)/3!(n-3)! ≥ 364 --> (n(n-1)(n-2)(n-3)!)/6(n-3)! ≥ 365
n(n-1)(n-2) ≥ 2190
Use trial and error. Use values starting from n = 11:
n | n(n-1)(n-2) | |
11 | 11(10)(9) | 990 |
12 | 12(11)(10) | 1320 |
13 | 13(14)(15) | 1716 |
14 | 14(13)(12) | 2184 |
15 | 15(14)(13) | 2730 |
For n = 15, the number of ways of choosing 3 random people became larger than 2190.
n = 15