Half life of a radioactive substance A is 4 days . The probability that a nucleus will decay in two half-lives is : - Clay6.com, a Free resource for your JEE, AIPMT and Board Exam preparation

After two half lives $\;\large\frac{1}{4}^{th}\;$ fraction of nuclei will remain undecayed . or $\;\large\frac{3}{4}^{th}\;$ fraction will decay hence the probability that a nucleus decays in two half lives is $\large\frac{3}{4}$