The half-life of a substance is the length of time for an unstable element to spontaneously decay to one half of its' original amount. Plutonium-239 has a half-life of 24 years. The formula for half-life is as follows: = 200(0.5)24 a) State the initial value. ________ b) State the decay factor. ________ c) Determine how long it would take for the sample to decay to only 5.0 g