There are three water inlet pipes A, B, and C in the reservoir. If you want to fill the whole pool with water, it takes 10 hours to open pipe A alone, 12 hours to open pipe B alone, and 15 hours to open pipe C alone. At 8:00 am, the three pipes were opened at the same time, and pipe A in the middle was closed for some reason. As a result, the pool was filled by 2:00 pm. Question: When was pipe A closed?
蓄水池有甲、乙、丙三个进水管．如果想灌满整池水，单开甲管需10小时，单开乙管需12小时，单开丙管需15小时．上午8点三个管同时打开，中间甲管因故关闭，结果到下午2点水池被灌满，问：甲管在何时被关闭？
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