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) (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`