Are the units in your answer compatible?
Checking units can expose an error before any detailed calculation. A printer producing 20 pages per minute needs five minutes for 100 pages, because pages divided by pages per minute leaves minutes. Multiplying those quantities would produce units that do not represent the time being asked for.
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Let the units check the operation
A printer's speed is twenty pages per minute. The task is to estimate the time needed for one hundred pages, ignoring setup and interruptions.
The calculation is 100 pages divided by 20 pages per minute, giving five minutes. The page units cancel, leaving the required time unit.
If you multiplied instead, the result would have units of pages squared per minute. That does not answer a question about duration.
Dimensional checking is especially useful when formulas contain several quantities. It cannot prove that every assumption is correct, and a dimensionally valid formula can still be wrong. But incompatible units reveal a definite problem that a plausible-looking number might otherwise conceal.