Don't think it's strange that Zhou Qun had 5,000 yuan. It's not strange at all. Jiang Lu had been covering all the household expenses, and Zhou Qun had been working for over a decade. He started working early, at fourteen or fifteen. Even though he had to support his family in the early years, after Jiang Lu married into the family, he stopped contributing to the household. Because he had a "good relationship" with his mentor's wife, he had connections and rose through the ranks quickly. His salary was high from the start, and he'd been getting raises over the years.
So 5,000 yuan was actually on the low side for him.
