Question

A radio isotope has a half life of 75 years. The fraction of the atoms of this material that would decay in 150 years will be.

• A. 66.6%
• B. 85.5%
• C. 62.5%
• D. 75%

Answer 1

Answer: (D)

75%

Answer 2

Number of half lives in 150 years $n = \frac{150}{75} = 2$

Fraction of the atom of decayed $= 1 - \left( \frac{1}{2} \right)^{n}$

$= 1 - \left( \frac{1}{2} \right)^{2} = \frac{3}{4} = 0.75$ ⇒ Percentage decay = 75%