Posted by Annie on Wednesday, September 19, 2012 at 3:46pm.
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, Wednesday, September 19, 2012 at 4:43pm
Since 1/8 = (1/2)^3, it will take 3 halflives, or 96 years.

Math  Reiny, Wednesday, September 19, 2012 at 5:03pm
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, Wednesday, September 19, 2012 at 5:34pm
ouch. How did I read 8% as 1/8?
As usual, go with Reiny's answer!
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