2004 · Q44
44. A set of 5 parallel lines is intersected by another set of 4 parallel lines so as to form a number of parallelograms. How many parallelograms can be formed?
MathematicsCombinatorics
- A12
- B20
- C40
- D60
Explanation
A parallelogram is determined by choosing any 2 of the 5 lines from one set and any 2 of the 4 lines from the other set. The number of ways is C(5,2) × C(4,2) = 10 × 6 = 60.