After x half-lifes, the amount of the isotope is:
A(x*T) = 90mg*e{-x*ln(2)}
And after 70 days, there are 5.628 mg
What is the half-life?
We define the half-life as the time such that the inital amount of a given substance reduces to half of it.
If the half-life is T, the amount of the substance can be written as:
A(t) = a*e^{-λ*t}
Where λ = ln(2)/T.
and a is the initial amount.
In this case, T = 17.5 days, then:
λ = ln(2)/17.5 days = 0.0396 / day
And we also have a = 90mg
Then the exponential equation is:
A(t) = 90mg*e^{-t*0.0396 / day}
After x half-lifes, the amount of the isotope left is:
A(x*T) = 90mg*e^{-x*T*ln(2)/T} = 90mg*e^{-x*ln(2)}
And after 70 days the amount is:
A(70 days) = 90mg*e^{-70 days*0.0396 / day} = 5.628 mg
If you want to learn more about exponential decays, you can read:
https://brainly.com/question/11464095
