Let the distance from D to A be X, the total freight is T, and the freight per kilometer of railway and highway is 3K and 5K respectively, where K is constant, X is variable and M is dependent variable.

Then: m =(20 squared +X squared) and then squared *5K+( 100-X)*3K. Simply put, it is a quadratic equation with one variable (parabola). If we find the lowest point according to the coefficients A, B and C, we can get the true value of X and finally get the result. I won't say much about the problem-solving process. The idea is this.