Meaning of "decline by"

Question

"By August, prices had declined by roughly one-third from their 1860 peak. "

If the price at 1860 was x, does that mean prices have become (1/3)*x or (2/3)*x ?

I know one-third means 1/3.

I just don't know how to interpret "decline by"

Answer 1

Decline with amounts or values means to become lesser, often slowly and with a connotation that it's not a good thing that it's becoming lesser.

A by prepositional phrase is used when you want to express "how much did X do" with most verbs.