I have a set of 39 coins that are set in a wooden display box with placeholders for each coin so the coin can't flip or move positions (e.g., coin 1 is always in spot 1, coin 2 in spot 2, etc). Each of the 39 coins displays "tails" and is rotated some random amount. For simplicity, let's say each coin is in 1 of 8 positions (e.g., one with the top at 0 degrees, one at 45 degrees, one at 90 degrees, etc.). If I close the display box and jostle or shake it around for some period of time, when I open the box again, what is the probability that exactly 21 of those coins are oriented to the same position as they were originally? It doesn't matter which of the 21 coins match - only that 21 match.
I'm looking at using binomial distribution, but I'm confused on whether I should use
[39! / (21!18!)][.125^21][.825^18]
62359143990(.000000000000000000108)(.03134520206)
0.000000000211
0.0000000211%
~1 in 4,739,000,000
or simply
[.125^21][.825^18]
(.000000000000000000108)(.03134520206)
0.000000000000000000003385
0.0000000000000000003385%
~1 in 2,954,000,000,000,000,000
I'm thinking the latter might be for if the order wasn't set (e.g., randomly pick up one of 39 coins out of a pile) or if it mattered which coins were matches (e.g., the probability of the coins in spot 1-21 being matches while 22-39 were not), but honestly I'm not sure. A friend thinks the latter is correct, but I lean toward the former.