I didn't read the article (used up my free reads and not subscribing), but my assumption from reading here was the fence in question is twice as long.
Given the assumptions listed at the beginning of my post, the answer is 8hr for the pair to paint a fence 2x as long.
The fence length is X.
Sarah can paint the fence in 6hr
John can paint a fence in 12hr.
The fence in question is twice as long, or 2X.
Sarah's speed is X/6hr
John's speed is X/12hr
Collectively they're painting X/6hr+X/12hr per hour which equals X/4hr
Thus:
2x=X/4hr
4hr*2x=X
8hrX=X
8hr
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