Notice in each case, the net amount traded is 27 dollars. Students pay 27, clerk gets 27. When the problem states that the clerk keeps two dollars, THE TWO DOLLARS ARE ALREADY INCLUDED IN THE 27 DOLLARS (25+2). So the last sentence has a trick. The clerk does not keep 2 more dollars on top of the 27. There is supposedly a 1 dollar difference between 29 (27 plus 2) and 30 (30 is the 8 initial amount of money handled but net amount handled is 27; 27 +3 -3). But the 30 is talking about the highest amount of the money exchanged, while 29 is a made up number.