The least-significant digit in the given measurement is the one in the 0.1 cm number place. The "greatest possible error" is half the value of that number place, so is ...
(0.1 cm)/2 = 0.05 cm . . . . "greatest possible error"
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For questions of this sort, we make the (not always practical) assumption that the measurement is rounded perfectly to the nearest 0.1 cm.
