Question

Half-lives of two radioactive substances A and B are respectively 20 min. and 40 min. Initially, the samples of A and B have equal number of nuclei. After 80 min. the ratio of the remaining numbers of A and B nuclei is-

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

Answer 1

Answer: (C)

1 : 4

Answer 2

Use, N = N₀/2ⁿ Ž N_A = $\frac { N _ { 0 } } { 2 ^ { 80 / 20 } } = \frac { N _ { 0 } } { 2 ^ { 4 } }$

N_B= $\frac { N _ { 0 } } { 2 ^ { 80 / 40 } } = \frac { N _ { 0 } } { 2 ^ { 2 } }$ Ž $\frac { \mathrm { N } _ { \mathrm { A } } } { \mathrm { N } _ { \mathrm { B } } } = \frac { 1 } { 4 }$

Ž N_A : N_B :: 1:4