The elder sister walks 70 meters per minute, and the younger sister walks 60 meters per minute. Half an hour after the younger sister set off, the older sister chased after her, and she was able to catch up in a few hours.
姐姐每分钟步行70米，妹妹每分钟步行60米．在妹妹出发半小时后，姐姐去追，多少小时后就能追上．
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