CHAPTER II

THE PROPORTION PRINCIPLE AND COMBINED OPERATIONS

10. Introduction. The ratio of two numbers a and b is the quotient of a divided by b or a/b. A statement of equality between two ratios is called a proportion. Thus

                              2/3 = 6/9,     x/5 = 7/11,      a/b = c/d

are proportions. We shall at times refer to equations having such forms as

                              2/3 = x/5 = 9/y = 10/z,     and     a/b = c/d = e/f

as proportions.

An important setting like the one for multiplication, the one for division, and any other one that the operator will use frequently should be practiced until it is made without thought. But, in the process of devising the best settings to obtain a particular result, of making a setting used infrequently, or of recalling a forgotten setting, the application of proportions as explained in the next article is very useful.