[ashishkabir]

D - 20? I'm not 100% so maybe someone can verify but here's my method:

To get to half the original concentration, it took from 1pm to 5pm = ie: 4hrs

We need the concentration to drop below 10

so, our calculation is as follows:

240 - 120 - 60 - 30 - 15 - 7.5

ie each dash is 4 hours long assuming this is first order kinetics

and since there are 5 dashes - 5 X 4 = 20 hours...