Q: After 280 days, the activity of a radioactive sample is 6000 dps. The activity reduces to 3000 dps after another 140 days. The initial activity of the sample in dps is

(A) 6000

(B) 9000

(C) 3000

(D) 24000

Ans: (D)

Sol: According to question activity reduces fro 6000 dps to 3000 dps in 140 days . Hence half life of Sample is 140 days .

In 280 days (or two half lives ) activity will remain 1/4 th of initial activity .

Hence , Initial Activity of the sample = 4 × 6000 = 24000 dps