26 A trauma center keeps ambulances at locations throughout the east side of a city in an attempt to minimize the response time in the event of an emergency.

The times, in minutes, from the ambulance locations to the population centers are given in the table at the top of the next column.

POPULATION CENTERS AMBULANCE LOCATIONS EAST NORTHEAST SOUTHEAST CENTRAL Site 1 12 8 9 13 Site 2 10 9 11 10 Site 3 11 12 14 11 Site 4 13 11 12 9 Find the optimal assignment of ambulances to population centers that will minimize the total emergency response time.