CAPE Physics Unit 2 · 2007 (Specimen) · Paper 1 · Question Q41

The decay constant of a radioactive sample of radon gas 1 \times 10^{-2}\text{ s}^{-1}. If there were 1.6 \times 10^5\text{ atoms} of radon gas present at a certain time, T, how much time must elapse before there is only 1.2 \times 10^5\text{ atoms} of radon left?

1. (A)25\text{s}
2. (B)29\text{s}
3. (C)45\text{s}
4. (D)400\text{s}