= Solution
`Kilometres` is an <ordinal categorical variable>: its five levels have a meaningful order. Treating it as an ordinary factor would use four <indicator variable>[indicator variables] and hence four unrelated <regression coefficient>[regression coefficients]. Converting the levels to numerical scores instead imposes a single linear trend across successive mileage bands. This is more parsimonious and gives greater precision when that trend is a reasonable approximation, although it also assumes that adjacent levels are equally spaced in their effect.
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