posted by Matt .
The age of a rock can be estimated by measuring the amount of 40Ar trapped inside. The calculation is based on the fact that 40K decays to 40Ar by a first-order process. It also assumes that none of the 40Ar produced by the reaction has escaped from the rock since it was formed.
40K + e- -> 40Ar k = 5.81 X 10^-11 year^-1
Calculate the half-life of the radioactive decay.