Question

The half - life of a radioactive element is 30 min. The time interval between the stages of 33.3% and 67.7% decay will be ?

• A. 60 min.
• B. 90 min.
• C. 30 min.
• D. 45 min.

Answer 1

The correct option is C

30 min.

Fraction left after first stage $=\frac{100-33.3}{100}=\frac{67.7}{100}=\frac{2}{3}$
Fraction left after second stage
$=\frac{100-67.3}{100}=\frac{33.3}{100}=\frac{1}{3}$
During the time interval of two stages, the element decays by 50%. Hence, time interval $=t_{\frac{1}{2}}=30$ min.