A human body excreates (removes by waste discharge, sweating, etc,) certain materials by a law similar to radioactivity.If technitium is injected in some form in 24 hours.a patient is given an injection containing ``^99Tc`.This isotope is radioactive with a half-life of 6 hours. The activity from the body just after the injection is `6 mu Ci`. How much time will elapse before the activity falls to `3 mu Ci`?