Radioactive decay question: The half-life of radioactive radium is 1620 years. What percent of a present amount of radioactive radium will remain after 100 years?

Question

anonymous · Answer

When you graph a curve relating to the half-life of a radioactive element/isotope, etc, what would the curve be?

anonymous · Answer

|dw:1360737987818:dw|

anonymous · Answer

Not quite. It's an exponential curve.

anonymous · Answer

But you didn't know this. I'm just giving you some insight on the curve. Now let;s get to the question. 
$$1620=100percent$$
$$100=?percent$$
Cross multiply.

anonymous · Answer

After you cross multiplied and found what the number the question mark equals, then you subtract that percentage from 100 to get your answer.

anonymous · Answer

So you don't use this formula at all? 
$$a=a _{0}e ^{kt}$$

anonymous · Answer

Why would you use that, when you can do a simple cross multiplcation and then subtract the percentage from 100 to get your answer?

anonymous · Answer

multiplication*