The correct option is B 65%
Let the initial amount of radioactive substance be No.
Using 0.693λt=lnNoN
After t=5 days, N=0.9No
∴ 0.693λ×5=lnNo0.9No .......(1)
Using 0.693λt=lnNoN
After t=20 days :
∴ 0.693λ×20=lnNoN ........(2)
Dividing equation (1) and (2), we get 520=lnN0.9NolnNNo
Or 14=0.104lnNoN
Or lnNoN=0.104×4=0.416
We get N=Noe0.416=0.65
Thus 65% of radioactive substance decays after 20 days.