In 1978, a 200-gram sample of cobalt-60 was buried in the Yucca Mountains, just northwest of Las Vegas, Nevada. How much of this sample was present in 2004? The half-life of cobalt-60 is 5.23 years.
1978 to 2004 is 26 years. Every 5.23 years, half of our 200-gram sample decays. It has reached its half life ~4.97 times. We use this formula to find our new amount:
[tex]200\times(\frac12)^\frac{26}{5.23}[/tex]
Every time it reaches its half life, it will be halved...the fractional exponent there is to pick up minute differences like in our 4.97.
