"Ratio and proportion" is discussed early in the course because most of the

problems dealing with pharmaceutical calculations can be restated or can be broken

down to a simple ratio and proportion problem. The principles discussed in this section

will be of great value in solving most problems.

a. Ratio: A ratio is the relationship of two quantities. A ratio may be expressed

as a ratio (1:8, 1:200, etc.) or as a fraction (1/8, 1/200, and so forth.).

b.

Proportion: A proportion is the equality of two ratios.

For example:

1

=

3

2

6

c. A check as to the equality of two ratios can be made by cross multiplying.

Multiply the numerator of the first ratio times the denominator of the second ratio. Then,

multiply the denominator of the first ratio times the numerator of the second ratio. If the

ratios are equal, the results of the cross multiplication will be the same.

d. The products of the cross multiplications are always equal in a proportion; if

one factor of either ratio is unknown, it may be solved for by substituting X for the

unknown factor in the proportion.

MD0802

1-35

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