i don't think you've learned about exponential decay as yet, neither do i think you would want to use it anyway. so i would say:

where is the initial amount and is the time

use the equation above and plug in the corresponding value for t

b. The person drinks the cup of coffee at 8 am and goes to bed at 11 pm Use f to predict the amount of caffeine in his bloodstream when he goes to bed.

1 cup has 100 mg of caffeine. 24 hours is 4 periods of 6 hours each. since 6 hours is the half-life and we don't want to use the original equation, we can find how much caffeine is in the guys body from the first cup of coffee after 24 hours by:

c. Suppose the person drinks another cup of coffee 24 hours after the first cup. How much caffeine will be in his bloodstream from these 2 cups of coffee just after drinking the second cup? Explain how you can find this result without using an equation?

100 * (1/2) * (1/2) * (1/2) * (1/2) = 6.25 mg

so after drinking a second cup, he will have 106.25 mg of caffeine in his blood stream