Final answer:
The usual dose of ibuprofen for an 18-month-old weighing 18 pounds would be approximately 40.8233 mg.
Step-by-step explanation:
To calculate the usual dose of ibuprofen for a child between 6 months and 2 years old, you need to convert their weight from pounds to kilograms. Since 1 kilogram is equal to 2.20462 pounds, you can use this conversion factor to find the weight in kilograms. In this case, the 18-month-old weighs 18 pounds, which is approximately 8.16466 kilograms.
Now, you can calculate the usual dose by multiplying the weight in kilograms by 5 milligrams per kilogram. So, for an 18-month-old weighing 18 pounds, the usual dose would be:
Usual dose = 8.16466 kg * 5 mg/kg = 40.8233 mg