Question

Bismuth-212 has a half-life of 60.5 minutes. Find the amount of bismuth-212 left from a 100-gram sample after 242 minutes

Answer 1

Notice that 242 = 4*60.5. This means after 242 minutes, the sample decays to $\dfrac1{2^4}=\dfrac1{16}$ of its original amount. So you end up with

$\dfrac{100\,\mathrm g}{16}=\boxed{6.25\,\mathrm g}$