Answer
6 minutes.
Work Step by Step
After 1 half-life, the activity=
$4000\,Bq\times\frac{1}{2}=2000\,Bq$
After 2 half-lives, the activity reduces to
$2000\,Bq\times\frac{1}{2}=1000\,Bq$
After 3 half-lives, it reduces to
$1000\,Bq\times\frac{1}{2}=500\,Bq$
3 half-lives are required for the activity to change from 4000 Bq to 500 Bq.
$t=3\times t_{1/2}=3\times124\,s=372\,s=6\,min$
