A a two-cycle engine runs on a mixture of oil and gas. A practical problem involves determining how many ounces of oil to add to a gallon of gas to get the right mixture. The problem is solving with proportion.
The number of bags of seed needed to cover a particular area, given the area one bag will cover, is calculated using a proportion.
A variation problem always involves two sets of information and two formulas. To solve a variation problem, a general formula is written from the variation statement. A specific formula can then be written with the constant of variation.
A specific formula is developed to solve a practical problem of converting scale length to real length in a scale drawing.