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

λ=0.693T1/2=0.69320=0.03465Now time of decay t=2.303λlogN0N⇒t1=2.3030.03465log10067=11.6 min and t2=2.3030.03465log10033=32 minThus time difference between points of time=t1-t2=32-11.6=20.4 min ≃20 min.