To determine how long Irwin was riding his bicycle, we can first calculate the time it took him to ride from one end of the trail to the other and then back.
- The length of the trail is 4 miles. Therefore, the total distance for the round trip is:
\[ \text{Total Distance} = 4 \text{ miles} \times 2 = 8 \text{ miles} \]
- Irwin's riding speed is 20 miles per hour. We can calculate the time taken to cover the total distance using the formula:
\[ \text{Time} = \frac{\text{Distance}}{\text{Speed}} \]
- Substituting in the values:
\[ \text{Time} = \frac{8 \text{ miles}}{20 \text{ miles per hour}} = 0.4 \text{ hours} \]
Thus, Irwin was riding for 0.4 hours.
The correct answer is 0.4 hours.