Can you please tell me how/when "in which" is used? This is a general question but I'll tell you where I see this. I see in most often as a modifier. If so, then when can it be used? Does the noun referrent need to be immediately preceding this modifier? "....., in which...." I haven't seen it today but I see it ALL the time and I'm not sure when it is allowed and when it is not.

Secondly, I did see "in which" used differently: Test Code (ETS) #37:
In the face of widespread concern about environmental waste, compact disc manufacturers are attempting to find a replacement for the disposable plastic box in which they package their product.

The correct answer to his question is A (repeating the sentence). Here "in which" is used differently. Can you please elaborate on how to use "in which"?

Thank you!

"in which" (or "with which," "by which," etc) is a special case in which the relative pronoun (which) is paired with a preposition (in this case, in). In English, we are not supposed to end a sentence with a preposition; instead, that preposition moves before the "which."

So the sentence you typed below (which it's okay to post since you posted only one sentence, even though it's from a paper test, which is a banned source) could be rewritten as:

CD manufacturers are attempting to find a replacement for the disposable plastic box which they package their product in.

But we can't have a sentence end with a preposition (in). Where to put it? Before the "which."

Here's a slightly simpler version:
Manufacturers found a replacement for the box which the product is shipped in.
Manufacturers found a replacement for the box in which the product is shipped.

Most people would say the first option, but the second one is grammatically correct. So, essentially, if you would want to put a preposition in at the end of the sentence (if you were to say it aloud to someone yourself), then pull that preposition just before the "which."

Also, note that many times "in which" is paired with "where" in GMAT sentences (that is, you have to choose between the two). Lots of people say "where" but technically that's supposed to refer to a physical or geographic location. So if whatever you're talking about is NOT such a location, don't use where.