Answer
$$4.1 mL$$
Work Step by Step
First we calculate the mass concentration of solution. To do so, we divide mass with volume: $100mg\div5ml=20mg/ml$
2. Next, we convert the body weight from lbs to kg. The conversion factor is 1 lbs = 0.45359 kg. $18lbs×0.45359lb/kg=8.16466kg$
3. Next we multiply the body weight in kg with mass of ibuprofen needed per kg to get the required mass of ibuprofen. $8.16466kgkg×10mg/kg=81.6466mg$
4. Finally, we divide the required mass of ibuprofen with its mass concentration to get the required volume of ibuprofen solution $81.6466mg÷20mg/ml=4.08233ml$
5. Round to two significant figures to get
$$4.1ml$$
