Where the missing dollar

The original problem:

Three guests check into a hotel room. The clerk says the bill is $30, so each guest pays $10. Later the clerk realizes the bill should only be $25. To rectify this, he gives the bellhop $5 to return to the guests. On the way to the room, the bellhop realizes that he cannot divide the money equally. As the guests didn't know the total of the revised bill, the bellhop decides to just give each guest $1 and keep $2 for himself.

Now that each of the guests has been given $1 back, each has paid $9, bringing the total paid to $27. The bellhop has $2. If the guests originally handed over $30, what happened to the remaining $1?

The followup solution:

A few months later, two of the original three guests check into a hotel room in the same hotel. The clerk says the bill is $20, so each guest pays $10. Later the clerk realizes the bill should only be $15. To rectify this, he gives the bellhop $5 to return to the guests. On the way to the room, the bellhop realizes that he cannot divide the money equally. As the guests didn't know the total of the revised bill, the bellhop decides to just give each guest $1 and keep $3 for himself.

Now that each of the guests has been given $1 back, each has paid $9, bringing the total paid to $18. The bellhop has $3, so $18 + $3 = $21, and the guests originally handed over $20, so that's where the missing dollar from the original problem is!