What is the origin and meaning of 30 years in Hedong and 40 years in Hexi?

It should be: thirty years to the east of the river, thirty years to the west of the river

It means: thirty years ago, Feng Shui was on the east side of the river, but thirty years later, it was on the west side of the river. Face. A metaphor for the changes in the world, the ups and downs.

Source: Chapter 46 of "The Scholars" by Wu Jingzi of the Qing Dynasty: "Mr. Da, 'Thirty years in Hedong, thirty years in Hexi'! Just like Thirty years ago, I saw with my own eyes what an advantage your two families had."