This is one of those questions meant to confuse you. There never was a $10 extra nor missing. The number $250 (the cost of the room adjusted) is not divisible by 3 (girls). So the answer really is the $10 is in the $250.
However, if you want it back, flirt with the guy and maybe he'll give you $10 to make it $240, meaning each girl paid $80 evenly and will get $20 back.