A migraine sufferer took Two 400mg doses of naproxen. The half-life for

this drug is 90 min. How much is left in her body after 9 hours?

If y (t) is the mass (in mg) remaining after t years, then y (t) = y (0) (0.5) ^{t/T} = 400 (0.5) ^{t / 4}, where T is the half-life period and y (0) is the amount at t = 0 years (initial).

Then at t = 20:

y (20) = 400 (0.5) ^{20 / 4} = / text{12.5 mg}