Function for a math problem (medicine dose problem)?

[Robert G]

The Question:
The doctor has prescribed a dose of 16 ml of "Cure-All".

Suppose that a dose of 16 ml is taken every four hours. How many ml of Cure-All will be present in your blood stream after the initial dose? (the body dissipates 1/4 of the total Cure-All it contains every four hours). The first repeated dose? The second repeated dose? The third?

Write a function for the amount of Cure-All that remains in your body over time.

so i have found the amount left after the first second and the third dose (12 ml 21 ml and 27.75 ml respectively) but i cannot find a function for this..

the pattern is this:
1st dose: 16(3/4) = 12
2nd: 16(3/4)+16(3/4)^2 =21
3rd: 16(3/4)+16(3/4)^2+16(3/4)^3 = 27.75

i just need a function for this pattern if it is possible..i cant seem to figure it out..thanks