Dr. Brown has a radio isotope source containing 1.2x10 to the 6th power atoms of plutonium238 & 1985. How many atoms will remain in 352 years?

The number of atoms remaining depends on the number you start with and the material's half-life, T_HL:


N(t) = N_0 (\frac 1 2 )^{(-t/T_{HL})}


The key is finding the value for T_HL for Plutonium - you can probably find that value in your text book.