I'm really stumped by this homework problem.

N(t) = No (1-e^(-0.0372t))

Where N(t) is final concentration at time t. No is final concentration.

How long does it take to get 85% of final concentration.

I can't figure out how to find the answer.

First I was thinking that N(t)= 85 and No=100, since I'm guessing it starts at 100% concentration.

85=100(1-e^(-0.0372t)) divide by 100

85/100=(1-e^-0.0372t)) take ln of both sides

ln(85/100) = 1-(-0.0372t) solve for t

t= (ln(85/100))/(1-(-0.0372)) = -0.156690593 which must be wrong, since its coming out negative

Then I though, maybe this is the right way to set it up.

N(t)= 85 (1-e^-0.0372t))

But then I have 2 variables, N(t) and t, when I really need to be solving for t.

I can't figure out what I'm supposed to use for N(t) and No values.

Unfortunately, there's no video walkthrough for this problem. And the class is online.

How do I figure out what the N(t) and No values are.