So the party of three (A Fighter - Dave, A Cleric - Marcy, and a Wizard - Mark) all roll up to a small town after fighting some goblins.
Weary, and in need of a long rest they pay the Inn Keeper's son 30 gold (each paying 10 gold from their recent battle!) and head up to the room.
The Inn Keeper came out, scolded his son, because it was The Holy Moon (Eclipse) and says it's only 25 gold. Hands his son 5 gold to give back.
As the son is walking back to the room, he realizes there's no even way to split 5 gold between the three party members, so he pockets two gold "for his troubles."
He gets to their room and hands them back, each 1 gold coin.
So now each of them has paid 9 gold, since they each got a 1 gold refund.
So 9 gold x 3 party members is 27 + 2 gold the Inn Keeper's son kept is... 29 gold.
So where is the missing gold coin?