Percentages Ratios and Proportions

Ratio = the relation between two similar magnitudes with respect to the number of times the first contains the second: the ratio of 7 to 2, written 7:2 or 7/2.

Proportion = comparative relation between things or magnitudes as to size, quantity, number, or ratio: 7/2 = 14/4

Percentage = a rate or proportion per hundred: 7/2 = 350%

Example:

If 5 tablets of Lipitor contain 20 grams of Lipitor, how many tablets are required to fill a prescription for 500 grams?

20 grams / 5 tablets = 500 grams / X tablets

4 grams / tablet = 500 grams / X tablets

X = 500 / 4

125 tablets

How many ml of solution containing 150 mcg of medication per ml must be administered to a patient that requires 1 milligram?

150 mcg / mL = 1 mg / X mL

150 mcg / mL = 1000 mcg / X mL (converted from mg to mcg)

X mL = 1000 / 150

X = 6.67 mL

How many grams of medication are required to make 100 mL of 1:5000 solution?

1 gram / 5000 mL = x grams / 100 mL

100 * (1/5000) = x grams

x = 0.02 grams