There is a straight road leading from the town of Willowbrook to Pinecrest 50 miles west and 15 miles south. Partway down the road, it junctions with a second road, perpendicular to the first, leading to the town of Greenfield. If the town of Greenfield is located 40 miles directly west of the town of Willowbrook, how far is the road junction from Willowbrook?
a) 25 miles
b) 30 miles
c) 35 miles
d) 40 miles