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

The correct answer is: 24000

Activity reduces from 6000dps to 3000dps in 140 days. It implies that half-life of the radioactive sample is 140 days. In 280 days (or two half-lives)activity will remain 1/4 th of the initial activity . Hence the initial activity of the sample is
4  6000 dps = 24000 dps