Suppose that a drug is administered to a person in a single dose, and assume that the drug does not accumulate in body tissue, but is excreted through urine. Denote the amount of drug in the body at timetbyb(t)and in the urine at timetbyu(t). If b(0)=3mgand u(0)=0mg, find a system of differential equations forb(t)andu(t)if it takes 46 minutes for the drug to be at one-half of its initial amount in the body.