In Genesis 42, Yosef's brothers, except Binyamin, come to him in Egypt. They don't recognize him, but he recognizes them — and accuses them of spying, and jails them. Then, in verses 18–20:
Yosef said to them on the third day: "Do this and live; I am god-fearing. If you are honest, your one brother will be stuck in your jail, and [the rest of] you go bring the food for the hunger of your houses and bring your little brother to me: your words will be verified, and you will not die." They did so.
"They did so"? They did what? They didn't have their brother stuck in jail: that was later (in verse 24). They didn't bring food back home: that was later (in verses 25 and on). They certainly didn't bring Binyamin back to Egypt: that was much later, in chapter 43. So "they did" what in 42:20?