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
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