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
60
15
30
45
Explanation:
N1 = 0.6 N0
N2 = 0.15 N0
$\dfrac{N_2}{N_1} = \left(\dfrac{1}{2}\right)^2$ so two half–life period has passed
So, time taken = 2t1/2 = 2 × 30 = 60 minutes.