Half-life of a radioactive substance is 20 minutes. Difference between points of time when it is 33%
disintegrated and 67% disintegrated is approximate

$\mathrm{\lambda }=\frac{0.693}{{\mathrm{T}}_{1/2}}=\frac{0.693}{20}=0.03465$

Now time of decay $\mathrm{t}=\frac{2.303}{\mathrm{\lambda }}\log \frac{{\mathrm{N}}_0}{\mathrm{N}}$

$$\begin{gathered} \Rightarrow {\mathrm{t}}_1=\frac{2.303}{0.03465}\log \frac{100}{67}=11.6 \min \\ \mathrm{and} {\mathrm{t}}_2=\frac{2.303}{0.03465}\log \frac{100}{33}=32 \min \end{gathered}$$

Thus time difference between points of time

$={\mathrm{t}}_1-{\mathrm{t}}_2=32-11.6=20.4 \min \simeq 20 \min .$