Let "d = 12 mi", "v = 60\\frac{mi}{h}".
Let the initial positions of the cars be "x_0" and "x_0 + d" respectively, and the first car move in the positive "ox" direction, while the other car in the negative. Hence, the equations of motion of the cars are "x_1(t) = x_0 + v t", "x_2(t) = (x_0 + d) - v t". When they pass each other, "x_1(t) = x_2(t)", from where "2 vt = d" and "t = \\frac{d}{2 v} \\approx 0.1 h = 6 min".
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