21. Bob was paddling his canoe upstream at at constant rate. After six miles, the wind blew his hat into the stream. Thinking that he had no chance to recover his hat, he continued upstream for six more miles before turning back. he continued rowing at the same rate on his return trip and overtook his hat at exactly the same spot where he began his journey eight hours earlier. What was the velocity of the stream?
23. The math department was making arrangements for a farewell luncheon to honor one of their retiring colleagues. They calculated that it would cost each person $30. "It's lucky that there aren't five fewer of us to split the bill, or it would be $10 more each", said one of the math teachers. How many people would be splitting the bill, and how much did the luncheon cost?
26. John was trying to take a short cut through a very narrow tunnel when he heard the whistle of an approaching train behind him. Having reached three-eighths of the length of the tunnel, he could have turned back and cleared the entrance of the tunnel running at 10 miles per hour just as the train entered. Alternatively, if he kept running forward, the train would reach him the moment he would jump clear of the tracks (at the exit.) At what speed was the train moving?
27. Ten years ago, a man was offered a motorcycle for $1024, but he declined to buy it. A year later, he was offered the same motorcycle for $640, but again decided not to buy it. A little while after that, he was again offered the motorcycle for $400. Again, he declined to buy it. Last week, he declined to buy the motorcycle again, even though it was now offered for $250. If the owner offers it for sale yet again, and he makes a consistent reduction of the price, how much will it be the next time he offers it for sale?
28. Susan is now four-fifths of her sister Jane's age. Three years ago, she was one-half her sister's age. Five years from now, Susan will be nine-tenths of Jane's age. If they are both under the age of ten, how old are they now?
29. Of three sisters named April, May, and June, none is yet twenty-one. April is now as old as June was fourteen years ago, and two-thirds of May's age. May, on the other hand, will be June's age when May is twice as old as she is now plus two years. Three years ago, May was as old as April is now. How old are April, May, and June?
30. For a wedding reception, the caterer found that 6 sandwich makers made 36 sandwiches in two hours. But, the caterer needs 216 sandwiches. How many sandwich makers does he need to make 216 sandwiches in one hour?