Question
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?Solution
For a parallelogram, we need 2 pairs of parallel lines. So, we can "choose" any two lines from the first set of 4 parallel lines 'and' choose any two lines from the second set of 5 parallel lines. Hence the value we need is 4C2 × 5C2 = 6 × 10 = 60 Thus 60 parallelograms are formed.
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