# 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