Question

If a radioactve substance decays for time interval equal to its mean life, then its fraction that remains undecayed will be

• A. $0.6322$
• B. $0.3678$
• C. $0.50$
• D. $0.75$

Answer 1

Answer: (B)

$0.3678$

Mean life is $halflife/0.693$

So fraction that remains undecayed wil be $1/2^{1/0.693}=1/2^{1.443}=0.3678$