Question

Half lives of two radioactive substances A and B are respectively 20 minute and 40 minute. Initially the sample of A and B have equal number of nuclei. After 80 minutes, the ratio remains in number of A and B nuclei is.

• A. 4:01
• B. 1:16
• C. 1:01
• D. 1:04

Answer 1

Answer: (D)

1:04

Answer 2

From the relation $\frac{N}{{{N}_{0}}}=\frac{1}{{{2}^{n}}}$ so $N\propto \frac{1}{{{2}^{n}}}$ Hence, $\frac{{{N}_{A}}}{{{N}_{B}}}=\frac{1/{{2}^{4}}}{1/{{2}^{2}}}=\frac{1}{{{2}^{4}}}\times \frac{{{2}^{2}}}{1}=\frac{1}{4}$