Answer:
8.27 minutes
Explanation:
Do the half lives first. 6400 Bq->3200 Bq->1600 Bq->800 Bq->400 Bq
The arrows represent one half life therefore there are 4. Next you multiply the 124 second with the 4 half lives. You get 496 second so you would divide that by 60 seconds which will give you 8.27 minutes.
