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 reduced to `((1)/(2)) xx ((1)/(2))` of the original number. The number of remaining active nuclei is, therefore,
`6.0 xx 10^(18) xx ((1)/(2)) xx((1)/(2))=1.5 xx 10^(18)`.