the half life of polonium is 139 days. you have a brand new sample of polonium that will not be useful to you after 80% of the material hasdistegraded. For how many days will you be able to use the polonium?

The half-life of Po is 139 days, so its decay rate is

0.5 = e^ (139k) = => k = ln (0.5) / 139

After some time t, 80% of the material will have decayed and you'll be left with a sample 20% of its original mass:

0.2 = e^ (kt) = => t = ln (0.2) / k = ln (0.2) / ln (0.5) * 139 = 322.748

so your sample is usable for about 323 days.