I can't for the life of me remember how to do natural log. I believe that's how you solve problems with e to some power. Anyways, here is the problem. It is from a section in my math book about surge functions. Any help would be greatly appreciated. I'm trying to finish all my homework in this class before it is due and this is the only assignment that's got me stuck.

If t is in hours, the drug concentration curve for a drug is given by C = 17.2te(to the, i.e.power) -0.4t ng/ml. The minimum effective concentration is 10 ng/ml.

If the second dose of the drug is to be administered when the first dose becomes ineffective when should the second dose be given.

Thanks in advance!

Frost