The starting amount of radioactive iodine-131 was 250 g. the half-life is 8 days. when leslie measured the amount, she measured 15.625 g of radioactive iodine. how many days have elapsed between measurements?

16
24
32
64

For this problem, we use the decay equation which is expressed as:

An = (Ao)e^-kt

where An is the amount left in t time, Ao is the initial amount and k is a constant.

We first calculate the value of k as follows:
An = (Ao)e^-kt
0.5 = e^-k(8)
k = 0.087

We calculate the time as follows:

An = (Ao)e^-kt
15.625 = 250e^-0.087t
t = 31.87 days