How much do I cut off an 8 1/3 fence post to shorten it to 5 3/4 feet?

(8 + 1/3) - (5 + 3/4) = (8 - 5) + (1/3 - 3/4) = 3 - 5/12 = 2 + 7/12

As 1 foot = 12 inches, you cut off

2 ft 7 inches.
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1/3 - 3/4 = 4/12 - 9/12 = -5/12