这两个属于介词+关系代词的用法,其实进一步说属于介词如何选择的问题,就是到底是用哪个介词+which的用法,一般而言“介词+关系代词”中介词的确定可以从两个方面确定：
1）介词由先行词决定,此时常可与关系副词互换如：
I still remember the day on which you left for USA.我还记得你离开美国的日子.用on是因为which代替先行词the day,从句就相当于you left for UAS on the day,而英语中在具体某一天前的介词用on,此时on which=when.
I will visit the university in which I worked five years ago.我会参观我五年前工作过的大学的.用in是因为which代词先行词the university,从句相当于I worked in the university five years ago.此时in which=where.
2）介词根据从句中的动词、名词或形容词决定,因为此时一般这个介词与从句中的某个词构成短语.这个结构不能换成关系副词.如：
Do you know the university about which they are talking?你知道他们在谈论的那所大学吗?此处选with是因为which代词先行词the university,从句相当于they are talking about the university.talk about sth讨论.,故选about.此处不能换成关系副词.
Forget the problem about which you are worried.忘记你担心的问题吧.此处选about,是因为be worried about是固定短语.此处不能换成关系副词.
其实简单来说到底选什么介词,你只要将先行词带入从句中就知道了,如：
The old man has two sons,____whom one is a university student.此处填什么介词好呢?你知道将two sons带入从句中变成 two sons one is a university student.就明白此处是指两个儿子中的一个,应该为One of the two sons,所以介词选of.
I arrived at a village,______which there lived a man of 100.此处填什么介词呢?
变换句子,将先行词带入从句中成为a village there lived a man of qoo.很明显这句话要表示在这个村里住着一个100岁的老人,所以介词选in.
这就是介词+关系代词的基本用法,说得比较粗糙,希望对你有点帮助.还有问题继续留言吧