We use the exponential decay formula to solve for the percentage of atoms in a sample of Plutonium-238 will remain radioactive after 263.1 years. We are given with a half-life of 87.7 years and so, we can get the decay constant: k = ln 2 / 87.7 k = 0.0079 A/A0 = exp(-kt) A/A0 = exp (-0.0079(263.1)) A/A0 = 0.125 or 12.5%
