Question

In a simple initially there are equal number of two radioactive isotopes $A$ and $B,3$ days later the number atoms of $A$ is twice that of $B$. If half life of $B$ is $1.5$ days, half life of $A$ is;

• A. $1.5$ days
• B. $3$ days
• C. $9$ days
• D. $12$ days

Answer 1

Answer: (B)

$3$ days

$\begin{array}{c}\text{Let initial number of atoms of}\\ \text{A and}B\text{be No}\\ \text{Given after,}3\text{days}\\ \text{A is twice of}B\\ \text{B half life is}1.5\text{days}\\ \therefore \text{After}2\text{half lives No/4}\\ \begin{array}{cc}{N}_A& =2\left(\frac{N_0}{4}\right)\\ N_A=\frac{N_0}{2}& \\ \text{Half life of A is 3 days.}& \end{array}\end{array}$