Exercise 3 solution

800 x 0.65 = 520 units. The knotted rope fails at about 520, not 800.

For a washing line, ease of untying (and ease of tying) matter most: the load is light, so strength is not the limit, but you want to take the line down without a struggle. Security matters too, but a light load rarely tests it.
Any reasoned choice is fine.