$\backslash$table$[[$From$/$to$,1,2,3,4,5,6],[1,0,10,20,30,30,20],[2,10,0,25,35,20,10],[3,20,25,0,15,30,20],[4,30,35,15,0,15,25],[5,30,20,30,15,0,14],[6,20,10,20,25,14,0]]$

There are six cities in region $R.$ The region must determine where to build fire stations. The region wants to build a minimum number of fire stations and ensure that at least one fire station is within $15$ minutes of each city. The travel times $($in minutes$)$ required to drive between cities are as follows. Develop a mathematical model to determine the optimal

location