What is one half of 23422?
One half of 23422 is 11711. Why on earth would you use wikianswers to divide by 2?
How many half lives would be necessary for a sample of parent isotopes to decay to the point that only one fourth of the sample is composed of parent isotopes?
I would take two half-lives for a radionuclide to decay to one fourth its original activity. In one half-life, it decays to one half. In the next half-life, it decays to one half of that one half, i.e. one quarter. In the next half life, it decays to one half of the one half of the one half, i.e. one eighth.