(1)

1x6=6

(2)

2x3=6

(3)

6=6

Therefore, the ratios are equal.

Because the products of the cross multiplications are always equal in a

proportion, if one factor of either ratio is unknown, it may be solved by substituting "X"

for the unknown factor in the proportion.

EXAMPLE:

1=X

2 6

1

X

2

6

Cross multiply:

2 (X) = 1 (6)

2X = 6

2X = 6

2 2

X=3

a. The numerators must have the same units.

b. The denominators must have the same units.

c. Three of the four variables must be known.

The expression of strength of a medication is usually the first ratio of the

proportion, and this ratio is put in proportion to the amount of medication to be

administered.

MD0904

2-3

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