A prescription for nifedipine calls for 0.2 mg/kg of body weight, four times per day. The drug is packaged in capsules of 5 mg. How many capsules per dose should be given to a patient who weighs 75 kg?
You can set the dose up as a ratio of "mg of nifedipine : kg of body weight"
$\displaystyle 0.2 : 1$
$\displaystyle 0.2 \times 75 : 1 \times 75$
$\displaystyle 15 : 75$.
So each dose, he is going to need $\displaystyle 15\,\textrm{mg}$ of nifedipine, or $\displaystyle 3$ capsules.