练习本3元一本，小云买了5本，付了x元，应找回______元．
应找回的钱数为：x-3×5=x-15．
故答案为：（x-15）．
野鸭从南海飞回北海需要7天，大雁从北海飞往南海需要9天，现在它们同时分别从南海、北海起飞，大约要______天会相遇．
— Will you be back ____ five in the afternoon?
— I'm not sure, maybe later than that. [ ]
A. in
B. before
C. for
D. until
