Easy Way to do this Problem?

Test Preparation AP Test Preparation
1

<p>For APES 2003, #93:</p>

<p>Uranium-235 has a half-life of 710 million years. If it is determined that a certain amount of stored U-235 will be considered safe only when its radioactivity has dropped to 0.10 percent of the original level, approximately how much time must the U-235 be stored securely to be safe?</p>

<p>(A) 7.1 x 10^6 years
(B) 7.1 x 10^7 years
(C) 7.1 x 10^8 years
(D) 7.1 x 10^9 years
(E) 7.1 x 10^10 years</p>

<p>Answer is (D). I don’t know how to do this.</p>

2

<p>3 half-lives means it will drop to 1/8 of its original level. 1/8= .125 which is closest to .1
3 half-lives means 3x7.1x10^8 = 2.1x10^9. I guess it’s the closest answer, especially given you might have to go through another half-life. Bad question imo. </p>

<p>A caveat might be that U-235 has radioactive products as well, so this might just be one of the things you have to know offhand.</p>