mba.py

# There are many ways to do this, but without knowing circumstantial constraints I chose simple/brute-force.

def activities_by_location(activities):
    locations = {}
    for k,v in activities.items():
        for location in v:
            if location not in locations:
                locations[location] = []
            locations[location].append(k)
    return locations

activities = {"skiing": ["Winter Park", "Park City", "Saalbach"],
              "surfing": ["Maui", "Arrifana"],
              "mountain biking": ["Winter Park", "Park City", "Moab", "Maui"]}

print(activities_by_location(activities))