A human body excretes (removes by waste discharge, sweating etc.) certain materials

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