student's home is 2,400 meters away from the school. He starts off at 7:18 every morning by bike, and arrives at the school just on time at an average speed of 200 meters per minute. One minute after he set off one day, he found that he didn't bring his math homework, so he went home immediately to get it. When he returns to school again, how many meters per minute can he travel at an average speed so as not to be late? (The time to take homework is 1 minute)
一个学生的家离学校有2400米，他每天早上骑车7：18出发，以平均每分钟200米的速度，刚好准时到校．有一天他出发1分钟后，发现数学作业没带，立即回家拿．当他返回到重新到校，平均得以每分钟多少米的速度才能不迟到？(拿作业的时间为1分钟)
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