Continuously compounded interest

[nrslmz]

Homework Statement

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.

Homework Equations

The formula for continuous compounding is
Mfinal = Minitial x e^(rt)

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.

 

[nrslmz]

ok I sorted it out. sorry to bother.