pre-calc problem?

Question

the total amount of a certain drug diminishes in the blood exponentially. if a patient receives 20 mg at noon and it is known that only 12 mg remain at 8 pm,

A) write an equation for the amount of the drug remaining after t hours.

B) find the amount of drug remaining in the patient's blood at noon the next day

i really just need an explanation or guidance with A. i'm bad at math okay?! i've been absent so i'm doing the classwork (meant to be done with help of teacher/classmates) at home and so i'm just checking ha.

Answer 1

12 = 20c^8
12/20 = c^8
(12/20)^(1/8) = c

Formula: A = 20(c^t) (put the value of c where the c is)

At noon the next day, t = 24

A = 20 [(12/20)^(1/8)]^24 = 4.32 mg

Answer 2

A = A1e^-kt
12= 20 e^-8k
.6 = e^-8k
ln(.6) = -8k
k = ln(.6)/-8 = .06385
A = 20e^-.06385t
A = 20 e^(-.06385*24) = 4.32 mg remaining after 24 hours