Question

Half-life of a radioactive substance A is two times the half-life of another radioactive substance B. Initially the number of nuclei of A and B are N_A and N_B respectively. After three half lives of A number of nuclei of both are equal. Then the ratio N_A/N_B is

• A. 1/4
• B. 1/8
• C. 1/3
• D. 1/6

Answer 1

Answer: (B)

1/8

Answer 2

Three half-lives of A is equivalent to six half-lives of B. Hence, N_A$\left( \frac { 1 } { 2 } \right) ^ { 3 }$= N_B $\left( \frac { 1 } { 2 } \right) ^ { 6 }$

or $\frac { N _ { A } } { N _ { B } }$ = $\frac { 1 } { 8 }$