To determine the rate of reading 21 chapters in 7 days, you can divide the total number of chapters by the number of days:
\[ \text{Rate} = \frac{\text{Total Chapters}}{\text{Total Days}} = \frac{21 \text{ chapters}}{7 \text{ days}} = 3 \text{ chapters per day} \]
So, the rate is 3 chapters per day.