Hey guys, I'm stuck on a problem:

The concentration C of a drug in the bloodstream decays exponentially according to the equation

C(t) = (D/V)(e^-kt)

where t is time, D is initial dosage in mg, V is the volume in litres and k is a rate constant.

A drug is said to be active if the concentration in the bloodstream is above some minimum concentration C(min). The duration of response t_duration is the amount of time for which a drug is active in the bloodstream.

I need to prove that the duration of response increases by one half life when the initial dosage is doubled.

Anybody know how I would approach this problem?