Question

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

• A. $6000$
• B. $9000$
• C. $3000$
• D. $24000$

Answer 1

The correct option is D $24000$

Here, the activity of the radioactive sample reduces to half in $140$ days. Therefore, the half life of the sample is $140$ days. $280$ days is it's two half lives. So before two half lives it's activity was

$2^6\times 6000=24000dps.$

Hence, the option $D$ is the correct answer.