In the previous activities, we constructed a number of tables. Once we knew the first numbers in the table, we were often able to predict what the next numbers would be. Whenever we can predict numbers in one row of a table by multiplying numbers in another row of a table by a given number, we call the relationship between the numbers a ratio. There are ratios in which both items have the same units (they are often called proper ratios). For example, when we compared the diameter of a circle to its circumference, both measured in centimeters, we were using a same-units ratio. Miles per gallon is a good example of a different-units ratio. If we did not specifically state that we were comparing miles to gallons, there would be no way to know what was being compared!
When both quantities in a ratio have the same units, it is not necessary to state the unit. For instance, let's compare the quantity of chocolate chips used when Mary and Quinn bake cookies. If Mary used 6 ounces and Quinn used 9 ounces, the ratio of Mary's usage to Quinn's would be 2 to 3 (note that the order of the numbers must correspond to the verbal order of the items they represent). How do we get this? One way would be to build a table where the second row was always one and a half times as much as the first row. This is the method we used in the first two lessons. Another way is to express the items being compared as a fraction complete with units:
<span>6 ounces
9 ounces</span>Notice that both numerator and denominator have the same units and thus we can "cancel out" the units. Notice also that both numerator and denominator have values that are divisible by three. When expressing ratios, we generally treat them like fractions and "reduce" or simplify them to the smallest numbers possible (fraction and colon forms use two numbers, as a 3:1 ratio, whereas the decimal fraction form uses a single number—for example, 3.0—that is implicitly compared to the whole number 1).<span>
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