Key learning points

1. Direct proportion can be used in a number of contexts, for example in currency conversion.
2. Proportional relationships can be expressed as ratios, often in unitary form.
3. A ratio is in unitary form if one of the parts is equal to one.
4. It is useful to be able to recognise which contexts involve direct proportion.

Keywords

• Rate - A rate is a relationship between two units. For example miles per hour (mph) is a rate called speed which relates distance and time.

• Exchange rate - An exchange rate is the rate at which one currency is exchanged for another. The two currencies are in proportion because they have a constant multiplicative relationship.

• Direct proportion - Two variables are in direct proportion if they have a constant multiplicative relationship.

Common misconception