Answer
$$(b) \space 90 \space mg/lb$$
Work Step by Step
$$m = 28 \space kg \times \frac{2.20 \space lb}{1 \space kg} = 62 \space lb$$ $$23 \times 80 \space mg \space acetaminophen = 1.84 \times 10^3 \space mg \space acetaminophen$$
$$\frac{1.84 \times 10^3 \space mg}{62 \space lb} = 30 \space mg/lb$$ $$30 \space mg/lb \div 8h = 3.75 \space mg/(lb \space h)$$ $$ 3.75 \space mg/(lb \space h) \times 24 \space h = 90 \space mg/lb$$
