Dimensional Analysis
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State
Dimensional analysis subjects a measurement's units to the same operations as its number, confirming every step of a calculation.
Show
Dividing 100 meters by 10 seconds gives both a number, 10, and a unit built the same way, m/s, in one operation.
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Skipping units and reattaching a plausible one at the end defeats the method's purpose — units must be carried through.