How Does Caffeine Decrease in Your Bloodstream Over Time?

[HelloMotto]

I'm having trouble with this question.

The percent "P", of caffeine remaining in your bloodstream is related to the elapsed time, "t', in hours, by t = 5(log P/ log 0.5)

a) How long will it take for the amount of caffeine to drop to 25% of the amount consumed?

b) Suppose you drink a cup of coffee after dinner at 5:00 pm. What % of the caffeine will remain in your system when you go to bed at 10:30 pm.

 

[Tedjn]

a) What would P be if the caffeine in your blood stream is now 25% of the original amount consumed?

b) What variable, t or P, do you know in this case? How would you solve for the other one?