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Explanation:

There are 5 parallel lines and there are another 'n' lines which are parallel but perpendicular to the first 5 lines.

∴ Each of the first 5 lines (say they are all horizontal lines) are perpendicular to each of the other n lines (say they are vertical lines).

To form a rectangle we need two horizontal and two vertical lines.

∴ Number of rectanges = (number of ways of selecting 2 horizontal lines) × (number of ways of selecting 2 vertical lines) 

⇒ 360 = 5C2 × nC2 

⇒ 360 = 10 × n(n - 1)/2 

⇒ 72 = n(n - 1)

⇒ n = 9

Hence, 9.

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