B Mary picked up a pencil and a piece of paper , saying , “Now , you want to earn money for the autumn camping trip , right?” Tom , her brother , nodded . He wasn’t able to go last year because he hadn’t earned enough money during the summer . This year he came up with a plan . He made a necklace for his mother that everyone admired . They were surprised to learn that Tom had made it for her . Why not make more necklaces and sell them? “OK , ” said Mary , “The first thing you need to do is to figure out (计算) how much you have now . ’’ “I have $20.00 in the bank , and $7.25 in my room , ” explained Tom . “That gives you a total of $27.25 , ” said Mary , “That is called your capital . The next thing you need to know is how much it will cost you to make each necklace . ” “Well , the pet shop wilt give me all the feathers the birds lose . I'll have to buy the beads (小珠子) and string , ” said Tom . Mary wrote on the piece of paper : CAPITAL $27.25 EXPENSES Feathers no coat String for lo necklaces $2.50 Beads for lo necklaces + $15.00 Total $17.50 “How much do you think you can sell such a necklace for?” asked Mary . “Hmm , ” Tom looked thoughtful and said , “There's a shop in town that sells almost the same kind of necklace for $ 7.50 . If I charge (要价) $ 6.00 , people will think that's a bargain . ” “OK . If you sell each necklace for $ 6.00 , you'll make a good profit (利润) on each then , ” said Mary . “Hurray!” Tom cried , “If I sell 2.0 necklaces , I'll have enough from the profit for the camping trip and $10.00 left over . ”