Let x mg be the amount of drug in the patient at time t.
Drug is injected into the patient at a rate of P mg per min
The kidneys of the patient remove the drug at a rate proportional to the amount of drug at time t.
At a particular point in time, the drug concentration in the patient remains constant. This constant value is 2.5 P mg
Show that dx/dt = P - 0.4x
Hence express x in terms of P and t