1.1.1—All physical quantities consist of a numerical magnitude and a unit

A measurement is written as number×unit, such as 3.0 m. The unit states the scale, while the number changes when the unit is changed.

Keep significant figures consistent with the measurement uncertainty and convert units before substituting into equations.

2.5 km is 2500 m; the physical distance is unchanged even though the numerical value differs.

A bare number is not a complete physical measurement, and adding quantities with incompatible units is meaningless.