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【简答题】
在某批次的某种灯泡中，随机地抽取 个样品，并对其寿命进行追踪调查，将结果列成频率分布表如下.根据寿命将灯泡分成优等品、正品和次品三个等级，其中寿命大于或等于 天的灯泡是优等品，寿命小于 天的灯泡是次品，其余的灯泡是正品. 寿命（天） 频数 频率 合计 （1）根据频率分布表中的数据，写出 、 的值； （2）某人从灯泡样品中随机地购买了 个，如果这 个灯泡的等级情况恰好与按三个等级分层抽样所得的结果相同，求 的最小值； （3）某人从这个批次的灯泡中随机地购买了 个进行使用，若以上述频率作为概率，用 表示此人所购买的灯泡中次品的个数，求 的分布列和数学期望.
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