Answer:
I will suppose that the actual question is:
Iodine-131 has a half-life of 8 days. How much would be left of an original g sample after x days?
Ok, a half-life means that after that time, the mass of the original sample is reduced to half.
So if we start it a quantity g of iodine-131, after 8 days, we will have g/2.
Also, remember that the decay is written as an exponential decay, then we will have:
A(x) = g*(r)^x
where:
A is the amount of the sample after x days, g is the initial amount of the material (such that A(0) = g) and r is the rate of decay.
We know that:
A(8) = g/2 = g*(r)^8
Now we can solve this for r:
g/2 = g*(r)^8
1/2 = r^8
(1/2)^(1/8) = r = 0.917
Then the amount of material after x days is given by:
A(x) = g*(0.917)^x