A quantity is a number with a unit, and the unit rides along through every step of a calculation. Centimeters cancel against centimeters, so 240 centimeters becomes 2.4 meters. Choosing a quantity, such as centimeters per second for a cart, is how you describe a build. A tape measure or a stopwatch sets how many digits to report. An expression like 3t + 15 has terms, coefficients and factors, and each one means something in the Loft.