31. It takes 9 minutes to walk from Dinding's home to school. One day, Dinding and his sister set out for school together. After walking for 4 minutes, Dinding realized he had forgotten his Chinese textbook and immediately went back home to get it, while his sister continued on to school.
Dinding went back home, got his textbook, and then headed to school. That day, Dinding was $\qquad$ minutes late to school compared to his sister.
Answer: 8