Question

A radio-isotope has a half- life of 5 years. The fraction of the atoms of this material that would decay in 15 years will be.

• A. 1/8
• B. 2/3
• C. 7/8
• D. 5/8

Answer 1

Answer: (C)

7/8

Answer 2

$\frac{N}{N_{0}} = \left( \frac{1}{2} \right)^{15/5} = \frac{1}{8}$ ⇒ Decayed fraction $= 1 - \frac{1}{8} = \frac{7}{8}$