The counting rate observed from a radioactive source at t=0 was 1600 counts per second and at t=8s it was 100 counts per second. The counting rate observed as counts per second at t=4sec will be

Activity, A=A02n      here n =tTh ⇒100=16002n ⇒n=4 ⇒tTh=4 ⇒Th = t4=84= 2 sec Since t = 8 sec, it means that in 8 sec there are 4 half lives.  So, after t = 4 sec, there are 2 half lives ∴n=tTh=42=2 Now, activity =A=A02n=160022=400 cps