Question

The half life period of a radioactive substance is 5 min. The amount of substance decayed in 20 min will be.

• A. 93.75%
• B. 75%
• C. 25%
• D. 6.25%

Answer 1

Answer: (A)

93.75%

Answer 2

Number of half lives in 20 min = n $= \frac{20}{5} = 4$

Fraction of material remains after four half lives $= \frac{1}{16}$

Hence fraction that decays $= 1 - \frac{1}{16} = \frac{15}{16} = 93.75\%$