Percentage

A question may require several linked operations: interpret a formula or balanced equation, convert units, rearrange an equation, use a ratio, calculate a percentage and finally round sensibly. Treating these as a sequence of small decisions is more reliable than trying to complete the whole calculation in one step.

Percentages compare a selected part with a whole. A longer problem may link all three, so a clear sequence is more reliable than trying to perform the entire calculation in one step.

A chemical equation gives a relationship between substances, but a question may supply a mass, an amount in moles, a gas volume or a percentage. Arithmetic connects the supplied information to the required quantity.