The half - life of a radioactive substance is 20 min.

N=N0e-t( N is the number of sample left, and N0is the number of sample at the beginning is decay constant which is, =ln2T)

According to the question, t2 is the time when 2/3 of the sample is decayed.

23N0=N0e-t2

According to the question, t1 is the time when 1/3 of the sample is decayed.

13N0=N0e-t1

Dividing above two equation with each other, we get

2=et2-t1

ln2= t2-t1

ln2= ln2Tt2-t1

t2-t1=T=20 min

Hence the correct option is B, i.e. 20 min