MATH help
posted by HM .
a certain radioactive isotope has a half life of approx 1,300 years .
How many years to the nearest year would be required for a given amount of this isotope to decay to 55% of that amount.
So I am not sure where to put the 1,300
1,300=2600*e^t
or A=x*e^1300

Assuming a mass of Ao at time t=0, at time t=1300, m = 1/2
That is, A = Ao * 2^(t/1300)
You see, that as t = 1300, we have 2^1 = 1/2 Ao
So, now we need to convert 2^n to e^n
2 = e^(ln 2)
A = Ao * (e^(ln 2))^(t/1300)
A = Ao * e^(t/1300 * ln 2)
A = Ao * e^(t/1875.5)
So, when A = 0.55 Ao, we have
.55 = e^(t/1875.5)
ln(.55) = t/1875.5
0.5978 = t/1875.5
t = 1121 years
Makes sense, since at t=1300 years, the amount will be reduced to 0.5 
Thank you Steve, thank you very much.