The question you opened
Family size, better value
A 2 litre bottle costs 3.60 dollars. The 3 litre family size costs 5.70 dollars.
The shelf calls the big bottle better value. How much more does it cost per litre, in cents?
10 cents
The small bottle works out at 1.80 a litre and the family size at 1.90. The bigger bottle costs 10 cents more for every litre in it.
Divide each price by its size. Three dollars sixty over 2 litres is 1.80 a litre. Five dollars seventy over 3 litres is 1.90 a litre. The family size costs 10 cents more per litre, so the shelf label is wrong, and buying three litres as a 2 litre plus half of another would cost less than the family bottle. The arithmetic is easy and the habit is the point.
Unit price is the only number that compares two packages, and it is almost never the number printed largest. Supermarkets in many countries are required to show it somewhere on the label, usually in small type at the corner, and it is worth learning where that corner is. The assumption being exploited here is real. Larger packs usually are cheaper per unit, and shoppers generalise the rule.
Retailers know this well enough that some larger sizes carry a premium, and the practice has a name in the trade. The rule holds often enough to be believed and fails often enough to be worth checking. Watch for two related tricks. Sizes that do not divide cleanly, like 750 millilitres against 1.25 litres, make comparison feel like work and discourage it. And unit prices quoted in different units on adjacent labels, one per 100 grams and one per kilogram, are technically compliant and practically useless.
The mental version is quick. Round the sizes to something friendly, divide, and compare. Three dollars sixty over 2 is under 2 dollars. Five dollars seventy over 3 is 1.90, which is also under 2. When two answers land that close, the smaller pack is worth a second look rather than an automatic pass.
Technique: Divide price by size