A field guide from EverydayCalc
Start with the denominator
A percentage is a relationship to a starting quantity. If a price rises from 80 to 100, the difference of 20 is one quarter of 80. If it falls from 100 to 80, the same difference is one fifth of 100. This is why percentage increases and decreases are not interchangeable. Write down the original quantity before deciding which percentage describes a change.
Normalize package quantities
Comparing a small package with a large one is easier when both use the same quantity unit. Convert kilograms to grams, liters to milliliters, or packs to items before entering the comparison. The lower unit price is a mathematical answer, not necessarily the better purchase: spoilage, quality, storage, and how much you actually need may make the smaller package more useful.
Weights express importance
A weighted grade multiplies each score by the importance assigned to it, adds those products, and divides by the total weight. The weights do not have to sum to one hundred when they are normalized. They do have to refer to the same set of scores. If an assignment is missing, decide how the institution treats it before calculating a result; the tool does not infer a grading policy.
Scale only linear relationships
A ratio tool is appropriate when one quantity changes in direct proportion to another. It is useful for diagrams, mixtures under a stated assumption, and simple allocation. It does not establish that a real process is proportional. Save the original ratio and chosen target with the result, then check whether indivisible items need rounding or whether the practical process needs another rule.