I'm needing some help in solving a word problem.?

Use the formula N = Ie^kt, where N is the number of items in termof the initial population I, at time t, and k is the growth constant equal to the percent of growth per unit of time. A certain radioactive isotope has a half-line of approximately 1100 years. How many years would be required for a given amount of this isotope to decay to 40% of that amount?