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How many points of intersection are there between 4 vertical lines and 5 horizontal lines?
Details and assumptions
The vertical (corresponding horizontal) lines are all parallel to each other.
The lines are distinct from each other.
The answer is 20.
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1 solution
Solution 1: Since parallel lines do not intersect, the vertical lines will not intersect with each other, and the horizontal lines will not intersect with each other.
Otherwise, each vertical line will intersect each other horizontal line. Hence, there are 4 × 5 = 2 0 points of intersection.
Solution 2: We can draw the following diagram, and count that there are 20 points of intersection.