9.6.1 Other Proportion Solutions
You can tackle problems involving direct proportion in many ways. Using a formula is not always the easiest way, as the following example shows.
Suppose that in a coffee club at work, a group of people share a carton of milk each day, but provide their own coffee. The carton contains enough milk for 12 cups of coffee. If the carton costs 72¢, how much should someone who has four cups of coffee with milk pay?
This person had of the milk, so the person should pay of the cost of the milk, which is 24¢.
Alternatively, you can work out the cost of milk for one cup of coffee as a first step. [ Note how the problem has been simplified by considering just one cup of coffee first. ]
Since there is enough milk for 12 cups, one cup will cost 72¢ ÷ 12 = 6¢.
So, four cups will cost 4 × 6¢ or 24¢.