[SOLVED] Exponential and Logarithmic Equations

Hey guys

So my question is about the application of these equations.

The actual question is

"A doctor places an overweight man on a strict diet to reduce his weight from 200kg to 80 kg. If he loses 3% of his weight every week, how long will it take him to reach his target weight?"

In the course material, we've only learned how to do these problems where the person is percentage is positive and the person is either earning money or a population is increasing. So I'm not sure how to answer a question in which a person's weight is *decreasing* by 3%.

The base formula I've used is A=P(1+i)^n

So A=80, P=200, i= -0.03? and you're solving for n which will give you the answer in n?

Can anyone shed some light for me? Any help would be much appreciated.