Grammar∣ in which

You can use “in which” as a precise way to introduce a relative clause after a noun that refers to a place or to a time. 

For example

    In my laboratory there is a blue cabinet where old equipment is stored. 
→In my laboratory there is a blue cabinet in which old equipment is stored. 


    The man sat at the computer where his old accounting files were stored. (place)
→The man sat at the computer in which his old accounting files were stored.

    It was a time when the old were revered as sages. (time)
→It was a time in which the old were revered as sages. (time)

Use “in which” to avoid ending a sentence with a preposition, especially in more formal writing. Ending a sentence with a preposition has become more accepted in some formal writing situations, but you will need to refer to your analysis of your audience’s expectations.

For example
This is the computer that the virus was discovered in. (Informal)
You can change it to
This is the computer in which the virus was discovered. (Formal)