Consider the 20 points of a 4×5 grid. You randomly choose two points from the 20 points. What is the probability that the two points belong to a horizontal or vertical line?
if the two points can be the same point, then the answer is (4+4)⋅2020⋅20=25
if the two points are different, then the answer is (4+3)⋅2020⋅19=719