they work hard in the promotion of economy 和 they work hard for the promotion of economy 的差別在哪裡？如果不好說明，請提供一些例句。

Work hard in- implies that they are integral part of production.
Work hard for- means on behalf of or for the hood of. They’re not doing the same work.


I worked hard in this class.
I worked hard for this class.

In the first sentence you probably think of a student whereas the second sentence maybe makes you think of an instructor. The student is working hard but the instructor is working hard to better the student.

In your text, the difference in prepositions shows a difference in scale of effort and/or results.

I hope that helps!