1 section is 640 acres.
(1/8)·(1/2)·(1/8)·(640 acres) = 5 acres
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The property description actually seems a bit bogus. While 1/128 of 1 section is 5 acres, it is not clear exactly which 1/128 of the section is being specified. This appears to be a math problem, rather than a surveying problem.
Usually quarter-sections are further divided into quarters. Any half piece will be specified according to a cardinal direciton (N, E, S, W), and any quarter piece will be specified using two cardinal directions (NE, SE, SW, NW). An eighth piece would need three directions for its specification, but would typically be described in terms of a half of a quarter (N half of NE quarter, for example), rather than one-eighth.
