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小明郊游,早上9时下车,先走平路然后登山,到山顶后又原路返回到下车处,正好是下午2时.若他走平路每小时行4千米,爬山时每小时走3千米,下山时每小时走6千米,小明从下车到山顶走了____千米(途中休息时间不计).

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    Having a teenager at home can be a real headache. Many teenagers smoke, drink and even fight. They often do something dangerous.

    Earlier studies have shown that teenagers are more likely (可能) to make irrational decisions than people from any other age group, including children and adults. Is it that teenagers are too young to tell right from wrong? Not really. So what's the reason?

    According to Laurence, a teacher from a US university, the reason is that teenagers care a lot about how their peers (同龄人) view them—that is “peer influence”.

    As children enter their teenage years, they spend more time with their friends and classmates, and also they care more about what they think of them. This makes teenagers make decisions without thinking about the costs.

    In a test, a group of teenagers were asked to play a video driving game. When they played with their friends watching around them, they took more chances and drove more carelessly because that would increase their possibility of winning. But when they played alone, they drove more safely.

    Why do peers have such a big influence on teens' behavior?

As Laurence sees it, a teenager's brain is like a car with a good accelerator (油门) but a weak brake (刹车). The “accelerator” is fully developed by teenage years while the “brake” is still not fully developed. When teenagers are watched by their peers, they usually push hard on the accelerator. With their weak brake, it is likely that they are going to end up in an accident.

    But the good news is that a violent (暴力的)teenager doesn't necessarily become a violent adult. About two-thirds to three-quarters of violent youth grow out of it. “They get more self-controlled.”