Two Ropes

Two fifty foot ropes are suspended from a forty foot ceiling, twenty feet apart. Armed with only a strong knife (and some strong biceps), how much of the rope can you steal? Assume you can safely fall a distance of ten feet, and you can only reach a height of seven feet when standing.

How does the answer change if the ropes are only thirty-five feet long?

Source: Internet newsgroup rec.puzzles, used as Ken's Puzzle of the Day 7/7/94.

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