Answer:
10 mph
Explanation:
Let x mph be Devontre rate uphill, then x+20 mph its his rate downhill.
1. Time uphill:

2. Time downhill:

3. If it took him 20 minutes (1/3 hour) longer to ride uphill than downhill, then

Solve this equation:

The rate cannot be negative, thus, x=10 mph (rate uphill).