Answer: The function is M(t) = 40mg*0.992^t
Step-by-step explanation:
Half life refers to the time that a given quantity to reduce to half its initial value.
This is usually a exponential decay, so we want a function of the form:
M(t) = A*r^(*t)
Were t is time, in this case, t is the number of days
A is the initial value
r is the rate of decrease, we will find that for exponential decays r must be a number between 0 and 1. If r is closer to 0, we will have a fast exponential decay, if r is closer to 1 we will have a slower exponential decay.
We have that the initial mass is 40mg, this is at t = 0.
M(0) = A*r^(0) = A = 40mg
So now we have the value of A.
Now, we know that in t = 84, the mass of the substance will be half its initial value, so it will be:
A/2 = 40mg/2 = 20mg
this means that:
M(84) = 40mg*r^(84) = 20mg
r^(84) = 20mg/40mg = 1/2
= 0.992
Then the equation is:
M(t) = 40mg*0.992^t