Sometimes we have to convert between different units. Suppose that a 60-inch piece of wood is attached to a 6-foot piece of wood. How long are they together? Obviously, we cannot find the answer to this question by adding 60 to 6, because the two figures are given in different units. Before we can add the two numbers, we have to convert one of them to the unit of the other. When we have two numbers in the same units, then we can add them.