1 . 某校高一年级学生利用暑假假期期间进行志愿者活动,为了解学生参加志愿活动的时间,随机抽取了200名学生进行调查,将收集到的做志愿者时间(单位:小时)数据分成
组:
,
,
,
,
,
,时间均在
内,已知上述数据的
百分位数为
.
![](https://img.xkw.com/dksih/QBM/editorImg/2022/7/17/0ce72b26-64b3-4416-9e22-ed30e7e38af2.png?resizew=226)
(1)求
的值;
(2)现从第二组,第四组学生中采用按比例分层抽样的方法取
人,再从
人中随机抽取两人,求两人来自不同组的概率.
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/6f8c4c029e552954bd493b49aeab82d5.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/a7ae66c5401deed7341470ca37800463.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/b434a9b41356aa5e866f7324dd2d5b43.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/731fffa7b63b5de570a88f814e0f8063.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ae71ae960741fb0eb1a4afe068cdd161.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/b1296bd53fdf4ff683c6c4138739c4ea.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7717f0502d30b59ed1a9508d293c7ab1.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c87a64b1cff227605294bc1be2adfb84.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/8aa86faa9bfef703aead8c2606684dc5.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/22d3bf727f6c78896544c7f6c8ff33ba.png)
![](https://img.xkw.com/dksih/QBM/editorImg/2022/7/17/0ce72b26-64b3-4416-9e22-ed30e7e38af2.png?resizew=226)
(1)求
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/280860dd039e1305a5ccc455f63e8223.png)
(2)现从第二组,第四组学生中采用按比例分层抽样的方法取
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/6f8c4c029e552954bd493b49aeab82d5.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/6f8c4c029e552954bd493b49aeab82d5.png)
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