Posted by Annie on .
The halflife of radioactive strontium90 is approximately 32 years. In 1961, radioactive strontium90 was released into the atmosphere during testing of nuclear weapons, and was absorbed into people's bones. How many years does it take until only 8 percent of the original amount absorbed remains?

Math 
Steve,
Since 1/8 = (1/2)^3, it will take 3 halflives, or 96 years.

Math 
Reiny,
I would say
.08 = (1/2)^(t/32)
ln .08 = (t/32) ln .5
t/32 = ln .08/ln .5 = 3.6438..
t = 116.6 years
rough check:
after 32 years  50% left
after 64 years  25% left
after 96 years  12.5% left , (which is 1/8 but not 8%)
after about 16 more years we would lose about 1/4 of what was left, leaving appr 3/4 of 12.5 or appr 9%
answer is quite plausible 
math  oops 
Steve,
ouch. How did I read 8% as 1/8?
As usual, go with Reiny's answer!