There are two situations. The first one is that the milk station is built between AB, and the distance from A to the milk station is X, so: s1= 20 * x+70 * (40-x)+60 * (100-x); The second type; If the milk station is built between BC and the distance from A to the milk station is X, then S2 = 20 * x+70 * (x-40)+60 * (100-x). Then compare the two, the second question is the same!