Let be the initial amount of Radium-221, and let be the current amount of Radium.

Since 85% decayed, that means that the current amount is 15% of the original amount.

Thus, this implies that

Since the decay is modeled by , we see now that

Now, k can be found using the half-life formula: , where is the half-life.

So our equation now has the form

Can you take it from here?

--Chris