Caffeine and chemistry question.

[argent]

Ok I need a little help on this here folks. I ahve a friend who needs to know this. We know that caffeine has an approx. 1/2 life of six hours, and a 200mg dose is taken. How to we factor in the necessary varibale to figure out mg/ml of concentration or more precisely mcg/ml. Thanks in advance to anyone who can help.