Right at the bottom. Hope this helped!
Humans are the closest relative I think
1 pound = 0.45 kg
Therefore
18 pounds = 18 x 0.45 = 8.1 Kg
But, the dosage is 5 mg per kilogram of the body.
Thus,
5 mg for each of the kg will be
5 x 8.1 = 40.5 mg
Therefore, 40.5 mg Ibuprofen for 18 pounds body weight