Question

A radioactive sample has a half-life of 10 min. What fraction of the sample is left after 40 min.

Answer 1

$$N=N_02^{-t/t_{1/2}}$$

$$\frac{N}{N_0}=2^{-t/t_{1/2}}=2^{-40/10}=0.0625=6.25\%$$