1. Calculate the reading rate:
\text{Rate} = \frac{2 \text{ chapters}}{40 \text{ minutes}} = \frac{1 \text{ chapter}}{20 \text{ minutes}}
2. Use the rate to find the number of chapters Juan can read in 100 minutes:
\text{Chapters} = \left( \frac{1 \text{ chapter}}{20 \text{ minutes}} \right) \times 100 \text{ minutes}
3. Simplify:
\text{Chapters} = 5
Answer: 5