I have a quantity of drug in the body, d, and every four hours a dosage, m, is injected. Taking into account a rate of elimination by the kidneys of 5% every hour, this dosage m needs to be such that every four hours the quantity of drug d is 100 g.

So when m=100, t=0, 4, 8, etc

Would the differential equation be

dd/dt=(m+d)-d/20 ?