Question

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

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

Answer 1

The correct option is A $93.75\%$

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

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

Fraction decays $=1-\frac{1}{16}=\frac{15}{16}=93.75$