Puzzles/Arithmetical puzzles/Hotel Refund/Solution
The men paid $27 dollars. The desk clerk has $25 dollars and the bell-hop pocketed $2.
The mistake is expecting that what the men paid and what the bell-boy kept to add up to what the men initially gave the desk clerk. In fact, it is the amount that the room effectively cost them, plus the amount they received back, that should add to $30.
The second puzzle is very similar: $25 is sitting with the clerk, $2 with the men, and $3 with the bellhop. That adds to the required $30.