The half-life of a radioactive substance is 20 minutes. The time taken between 50% decay and 87.5% decay of the substance will be

Response:

20 minutes

30 minutes

40 minutes

10 minutes

In 20 min 50% of the substance decays and 50 % of substance left.
In next 20 min 50% of the remaining substance decays. 
In 40 min, $50+\frac{50}{2}=75\%$ of the substance decays, leaving 25% of the substance.
In next 20 min 50 % of the remaining substance decays.
In 60 min, $75+\frac{25}{2}=87.5 \%$ of the substance decays. 
So the time taken between 50% decay and 75% decay is 40 min.