Question

1. The half-life of an isotope is 5 years. What fraction of the isotope will remain after 20 years?

2. The half-life of Bi-210 is 5 days. What percentage of a sample of Bi-210 will remain after 10 days?

Answer 1

1.

$$\frac{N}{N_0}=2^{-t/t_{1/2}}=2^{-20/5}=1/16$$

2.

$$\frac{N}{N_0}=2^{-t/t_{1/2}}=2^{-10/5}=0.25=25\%$$