Explanation:
the dosage information bases only on age, on weight and the level of the fever. but we have no information about the fever.
so, we assume the fever is below 102.5° F.
the age is with 18 month in the valid range (0.5 to 2 years old) for the dosage.
so, all is left is to do the calculation about the weight.
24 pounds is based on the given information (1 kg ≈ 2.2 pounds) :
24/2.2 = 10.909090909... kg
this is rounded to full kg : 11 kg
the child should get 5 mg per kg, so this child has to get
5×11 = 55 mg