Units and the Four Basic Operations

• Addition and Subtraction:

  Units work in the same way for addition and for subtraction. The basic rule is that you can only add or subtract quantities measured in the same units. Or, more colloquially -- You can't add apples and oranges. It is possible, however, to add two quantities of the same kind that were measured using different units by first converting them to the same units. For example,

  2 feet + 4 inches = 28 inches

  or

  2 feet + 4 inches = 2 1/3 feet

• Multiplication and Division:

  For multiplication and division units follow the usual algebraic rules -- for example,

  2 feet * 4 feet = 8 feet²

  and

  

  The units feet² or square feet measure area. A region whose area is one square foot has the same area as a square that is one foot by one foot.

  The units feet/seconds or feet per second measure velocity. For example, a car moving at 50 feet per second will travel 200 feet in four seconds --