Question

What percentage of original radioactive atoms is left after five half lives.

• A. 0.3%
• B. 1%
• C. 31%
• D. 3.125%

Answer 1

Answer: (D)

3.125%

Answer 2

Fraction of atoms remains after five half lives

$\frac{N}{N_{0}} = \left( \frac{1}{2} \right)^{t/T} = \left( \frac{1}{2} \right)^{5T/T} = \frac{1}{32}$.

⇒ Percentage atom remains

$= \frac{1}{32} \times 100 = 3.125\%$