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| | Significant Figures |
 | | The number of significant figures in a measurement, such as 2.531, is equal to the number of digits that are known with some degree of confidence (2, 5, and 3) plus the last digit (1), which is an estimate or approximation. |
 | | There are four significant figures in both the mass of the penny (2.531) and the number of grams in a pound (453.6). |
 | | One way of rounding off involves underestimating the answer for five of these digits (0, 1, 2, 3, and 4) and overestimating the answer for the other five (5, 6, 7, 8, and 9). |
| chemed.chem.purdue.edu /genchem/topicreview/bp/ch1/sigfigs.html (547 words) |
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