Recently a survey has been done in Guangming Middle School about how students pay for goods． It's found that fewer students pay cash（现金） for goods and more students use their mobile phones to pay by scanning（扫描） QR code（二维码） through WeChat or Alipay． In 2016， about 70% students pay in cash and the rest by QR code scanning． However， in 2017， the students who use QR code rise sharply to 70%．

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    Xiao Ming is counting his money and he finds that he spent 100 yuan totally last week． He remembers that he paid ￥20 for breakfast， ￥15 for drinks and ￥10 for snacks． And he bought two pens for ￥6， two notebooks for ￥18 and three magazines for ￥20． All these cost him ￥89． Where is the missing 11 yuan？ Oh， it went to a medium size of potato chips and a small size．

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