The half-life of a radioactive substance is 30 minutes. The time (in minutes) taken between 40% decay and 85% decay of the same radioactive substance is

N0= Nuclei at time t=0N1= Remaining nuclei after 40% decay   =(1-0.4)N0=0.6N0N2= Remaining nuclei after 85% decay Hence, two half life is required between 40 % decay and 85 % decay of a radioactive substance.∴ Time taken =2τ1/2=2×30 min=60 min