Discussion
A set of 4 parallel lines intersect with another set of 5 parallel lines. How many parallelograms are formed?
Answer: C
The number of parallelograms is 60.
Each parallelogram is bounded by two lines from the first set and two lines from the second. The number of ways to choose them is
\[ \binom{4}{2} \times \binom{5}{2} = 6 \times 10 = 60 \]
20 is the number of intersection points (4 × 5), and 48 and 72 do not come from choosing pairs of lines from each set.
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