Question

Number of nuclei of a radioactive substance are $1000$ and $900$ at times $t=0$ and time $t=2s$. Then, number of nuclei at time $t=4s$ will be

• A. $800$
• B. $810$
• C. $790$
• D. $700$

Answer 1

The correct option is B $810$

Every $2s10$% of the substance decays, i.e.
After $2s$ there are $900$ nuclei, after $2$ more seconds, it must reduce by $10$%, which means $0.9\times 900=810$ nuclei would remain.