SOLUTION: in a plane, a set of 15 parallel lines intersects another set of 20 parallel lines to form parallelograms. find the number of parallelograms formed.

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Question 734102: in a plane, a set of 15 parallel lines intersects another set of 20 parallel lines to form parallelograms.
find the number of parallelograms formed.

Answer by KMST(5328) (Show Source):
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With so many parallel lines the number of parallelograms formed is very large.
for each parallelogram, two of the sides will be part a pair of parallel lines from the set of 15 parallel lines, and the other two sides will be part aother pair of parallel lines from the set of 20 parallel lines.
From the set of 15 parallel lines we can make
From the set of 20 parallel lines we can make
Since each of the choices for one pair of sides can be paired with any of the choices for the other pair of sides, the total number of parallelograms formed is