When Ibuprofen is given for fever to children 6 months of age up to 2 years, the usual dose is 5 milligrams $(\mathrm{mg})$ per kilogram $(\mathrm{kg})$ of body weight when the fever is under 102.5 degrees Fahrenheit. How much medicine would be usual dose for a 18 month old weighing 23 pounds?
\[
1 \mathrm{~kg}=2.2 \text { pounds }
\]
milligrams

Step 1 ：Understand the problem: The problem is asking us to find out how much Ibuprofen should be given to a child who is 18 months old and weighs 23 pounds. The dosage is 5 milligrams per kilogram of body weight.

Step 2 ：Convert pounds to kilograms: We know that 1 kilogram is equal to 2.2 pounds. So, we can set up the following equation to find the child's weight in kilograms: \(23 \text{ pounds} \times \left(\frac{1 \text{ kg}}{2.2 \text{ pounds}}\right) = x \text{ kg}\)

Step 3 ：Calculate the child's weight in kilograms: \(23 \text{ pounds} \times \left(\frac{1 \text{ kg}}{2.2 \text{ pounds}}\right) = 10.45 \text{ kg}\)

Step 4 ：Calculate the dosage: We know that the dosage is 5 milligrams per kilogram of body weight. So, we can set up the following equation to find the dosage: \(5 \text{ mg/kg} \times 10.45 \text{ kg} = y \text{ mg}\)

Step 5 ：Calculate the dosage: \(5 \text{ mg/kg} \times 10.45 \text{ kg} = 52.25 \text{ mg}\)

Step 6 ：Simplify the result: The result is already in its simplest form.

Step 7 ：Check the result: The result meets the requirements of the problem. The dosage of Ibuprofen for the child is 52.25 milligrams.

Step 8 ：\(\boxed{52.25}\) milligrams is the usual dose of Ibuprofen for an 18 month old child weighing 23 pounds.