Radium-221 has a half-life of 30 seconds. How long will it take for 85% of a sample to decay?
Ok so I really don't know what to do here. I know I can find r because its but what about the 85%? Any help is appreciated.
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