at which一問

This is a typical example of relative clause with preposition.

"banks lend to each other at the interest rates " is the sentence intended to define the "interest rates" in the main clause. There are many types of interest rates, so we have to be specific.

When transforming the above sentence into a defining relative clause, we have to keep the preposition "at" and place it before the relative pronoun "which".

Therefore, it becomes ... " at which banks lend to each other."

內容講到的banks lend to each other所指的是interest rate, 所以which是指這interest rate，而banks借款的interest rate是用lend at an interest rate of xx%.所以用at which.個preposition是依據句子中which所指的那東西所配合的。所以你會見到不同的句子內容會用上in which(一般指一樣固定地點/事態)，of which(一般指屬於某事/某人)，by which(由某事/某人引致)…等等