Calculus question, exponential growth and decay...

Assume that the amount, D(t), of drug in the body at time t, given by D(t) = Doe^-kt where k>0, models the elimination of a drug from the body. If a patient receives a 30 mg dose of a drug and if 1/4 of the dose is eliminated in 3 hours after being administered then the decay rate k (k>0) is:

A) -1/3ln(1/4)
B) -1/3ln(3/4)
C) -1/3ln(2/15)
D) -3ln(3/4)
E) -3ln(1/4)

I'm pretty sure it is B but can someone please confirm and show me how to do this question?

Thanks.

PS the "Do" is D knot.

From the info given, we have

30(3/4) = 30e^(-3k)

Since 1/4 has been eliminated, we still have 3/4.

Divide out the 30,

3/4 = e^(-3k)

ln(3/4) = -3k

k = ln(3/4)/(-3) = (-1/3)ln(3/4)

The answer is B)