A human body excretes ( removes by waste discharge sweating etc.) certain materials by a law similar to radioactivity. if technitium is injected in some form in a human body.the body exerts half - the amount in 24hours . A patient is given an injection containing 99tc this isotope is radioactive with a half-life of 6 hours.the activity frim the body just after the injection is 6μCi How much time will elapse before the activity falls to 3μCi?
