Hi there!
Use the formula d / s = t to solve for the time needed for each:
Distance = 20 miles
(J)
20 m / 10 mph = 2 hours
(B)
20 m / 8 mph = 2.5 hours
Subtract the times to find the amount of time necessary to wait:
2.5 - 2 = 0.5 hours or 30 minutes.