A radioactive sample has `6.0 xx 10^18` active nuclei at a certain instant. How many of these nuclei will still be in the same active state after two half-lives?

Explanation

In one half-life the number of active nuclei reduces to half the original number. Thus, in two half lives the number is reduces to `((1)/(2))xx((1)/(2))` of the original number. The number of remaining active nuclei is, therefore , `6.0xx10^(16)xx((1)/(2))xx((1)/(2))=1.5xx10^(18)`