There are 20 lines on a plane. Five of them are parallel to one another. The lines divide the plane into regions. Find the greatest possible number of regions.

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We have a geometry problem about line arrangements. There are 20 lines on a plane, with 5 of them being parallel to each other. We need to find the maximum number of regions these lines can create when they divide the plane. Let's start by understanding what we have: 5 parallel lines and 15 additional lines that we can position optimally.