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Correct answer: 101
- Classify the 20 lines
There are two groups:
-
Odd-indexed lines:
These are distinct lines, all parallel to each other.
-
Even-indexed lines:
These are distinct lines, all passing through a fixed point .
We want the maximum number of distinct intersection points of pairs of lines.
- Intersections among odd-indexed lines
Since all odd-indexed lines are parallel, no two of them intersect.
So, number of intersection points from this group:
- Intersections among even-indexed lines
All even-indexed lines pass through the same point .
Any pair of these lines intersects at , but this gives only one distinct intersection point.
So, number of distinct intersection points from this group:
- Intersections between an odd-indexed line and an even-indexed line
Each odd-indexed line is parallel to the other odd-indexed lines, and each even-indexed line passes through .
To maximize the number of distinct intersection points:
- none of the odd-indexed lines should pass through ,
- no even-indexed line should be parallel to the odd family.
Then each odd line intersects each even line in exactly one point.
Number of such pairs:
Now check whether these intersection points are distinct.
- For a fixed even line, its intersections with the parallel odd lines are distinct.
- For two different even lines, their intersection with a given odd line are different because the even lines meet only at , and we ensured the odd lines do not pass through .
Hence all these points can be made distinct.
So, number of distinct intersection points from mixed pairs:
- Total maximum number of distinct intersection points
Adding all contributions:
- Final Answer
The maximum number of points of intersection is
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