Question

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?

Answer 1

Answer:

The probability of both points falling in the same row or column is 7/19, or approximately 37%

Step-by-step explanation:

The easiest way to solve this is to think of it rephrased as "what is the probability that your second point will be in the same row or column as your first point".  With that frame of reference, you can simply consider how many other points are left that do or do not fall in line with the selected one.

After selecting one, there are 19 points left.

The row that the first one falls in will have 3 remaining empty points.

The column will have 4 remaining empty points.

Add those up and you have 7 possible points that meet the conditions being checked.

So the probability of both points falling in the same row or column is 7/19, or approximately 37%