Best Buys uses proportion to compare two different proportions of the same item. Determine the unit cost for each of the two proportions, then determine which gives better value for money.
The unitary method can also be used to work with more appropriate values. Using values of 100s, 1000s, etc, may make calculations easier in some instance, such as working out the cost of 100g of a product.
1. Value brand coffee costs £1.95 for 200g. A premium brand is on offer which is 300g for £2.89. Which works out to be cheaper?
Answer: Premium brand
Value brand: 200g costs £1.95, or 195p.
This is a proportion of 200 : 195, change to 100 : 97.5
Premium brand: 300g costs £2.89, or 289p.
This is a proportion of 300 : 289, change to 100 : 96.3
The premium brand is actually cheaper, as it costs 96.3p for each 100g.
2. In a shop, safety razors for men cost £9.99 for a pack of four. Safety razors for women cost £7.99 for 3. Which are more expensive?
Answer: For women
Men: 4 cost 999p, which is 4 : 999 or 1 : 249.75.
Women: 3 cost 799p, or 3 : 799 which is 1 : 266.33
The unit cost of the razors for women are higher.