1. The problem statement, all variables and given/known data I must give medication to a patient continuously, but at the same time the kidneys eliminate the 2,5 per cent of the medication. I need to have 90micrograms of medication at the end of 4 hours. The initial medication amount is zero. Normally, the limit of the compound intereset formula must be taken. 2. Relevant equations The formula for continuous compounding is Mfinal = Minitial x e^(rt) 3. The attempt at a solution Mfinal is 90, r is -0,025, t is four and I need to find Minitial. Or is r 0,975. I try to solve the equation. I feel that I must find the rate first, like ln(Mfinal/Minitial)/t; but I can't without the initial value. Please help.