A car travels 240 km on 12 litres. A learner reports 20 litres per km after calculating 240 divided by 12. Which label corrects the unit-rate error?

Explication

The calculation divides kilometres by litres, so its result must be labeled 20 km per litre. Calling it litres per km reverses the units; that reciprocal rate would require dividing 12 by 240 instead.