Set Covering

The U.S. government office of homeland security decides to open offices for special "disaster emergency crews". The goal is to have each major U.S. city be within 120 miles of one of these special offices. Suppose the only major U.S. cities under consideration were, New York, Philadelphia, boston, Hartford, Washington D.C. and Baltimore. How many offices do they need to open, and where should they go?
New York Philadelphia Boston Hartford Washington D.C. Baltimore
New York 100 220 90 210 170
Philadelphia 100 310 180 110 80
Boston 220 310 120 430 370
Hartford 90 180 120 290 250
Washington D.C. 210 110 430 290 45
Baltimore 170 80 370 250 45