Question

A radioactive sample has 2.0 x 10${}^{20}$ active nuclei at a certain instant of time. Then the the number of samples in the same active state after three half-lives is

• A. 2.5 x 10${}^{15}$
• B. 2.5 x 10${}^{21}$
• C. 2.5 x 10${}^{19}$
• D. 2.5 x 10${}^{16}$

Answer 1

Answer: (C)

2.5 x 10${}^{19}$

After 3 half lines, the no. will be ${\frac{1}{8}}^{th}$ of its initial value.

i.e. $=\frac{1}{8}\times 2\times {10}^{20}$

$=2.5\times {10}^{19}$