# Math Help - Half-life

1. ## Half-life

The half-life of radioactive strontium-90 is approximately 29 years. In 1963, radioactive strontium-90 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 7 percent of the original amount absorbed remains?

2. Originally Posted by noles2188
The half-life of radioactive strontium-90 is approximately 29 years. In 1963, radioactive strontium-90 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 7 percent of the original amount absorbed remains?
The fraction remaining after $t$ years is $2^{-t/29}$, so you need to solve:

$0.07= 2^{-t/29}$

CB

3. I would have written it $\left( \frac{1}{2} \right)^{\frac{t}{29}} = 0.07$ but that's exactly the same thing.